How much do you know about the beautiful blue marble we call home? From how our planet formed to just how many species there are, here are 100 things you need to know about Earth and life on Earth, adapted from an episode of The List Show on YouTube.
Earth is the only planet in our solar system that doesn’t get its name from Greek or Roman mythology.
Eorþe means “ground, soil, dry land.” The same term was also used to differentiate between our domain, the underworld, and the heavens. This was before our modern conception of planets. The fact that we needed a way to refer to our planet before we even really figured out what a planet is probably helps account for the murky history around the word. It’s impossible to say who first used the word earth, or one of its antecedents, to refer to the astronomical entity that is the earth. One usage probably just bled into another as our scientific understanding expanded.
Earth is in the Milky Way galaxy, the third planet from the sun in our solar system.
According to NASA, it takes about eight minutes for light from the sun to reach us on Earth.
That’s not some great coincidence—historically, an astronomical unit was just the average distance from the Earth to the sun. It gave scientists a shorthand for communicating distances. New York to California is about .00003 astronomical units.
Maps have been around for a long time. The oldest surviving maps are thousands of years old; there’s thought to be a map of the stars found in the Lascaux caves of France that dates back to 14,500 BCE.
There also might be a map of local landscape features of what is now the Czech Republic carved into a mammoth tusk that dates back to 25,000 BCE. The key is “might be,” because it can be a bit tricky, when looking at millennia-old carved tusks, to differentiate between abstract drawings and intentionally plotted maps. But there are proponents of the 27,000-year-old tusk map.
When they were invented around 1760, jigsaw puzzles were used to teach kids about geography, fitting the various countries together in order to make a map of the world (or, more commonly, just Europe).
The earliest surviving map of the world is the Babylonian World Map, which dates back to the 6th century BCE. The clay tablet contains a labeled depiction of the known world, centered around the Euphrates River. It’s currently housed in the British Museum.
The Babylonian World Map wasn’t very accurate, but even modern maps aren’t perfect—far from it. The common map we all probably have in our heads is based on the Mercator projection. This map preserves the shape of land masses, but stretches the area of regions closer to the poles. It’s been the standard model of maps for centuries, but it’s mainly beneficial for sailing, not accurately depicting the relative size of objects.
Because of the size distortion of Mercator projections, certain countries like Greenland are depicted as being massive. If you picture Greenland in your head, it’s probably almost the size of Africa—but in reality, Greenland is about the size of the Democratic Republic of the Congo. About 14 Greenlands could fit inside of Africa.
The Mercator projection inflates the apparent size of Antarctica to such a degree that most publishers choose to cut it off, which often results in world maps with Europe near the vertical center. The equator can end up about two-thirds of the way down the map, even though “being in the middle of the world” is kind of the equator’s thing.
Some see an implicit political statement arising from these distortions. When Boston public schools decided to stop purchasing Mercator projection maps for their classrooms, an official framed it as part of an effort to move past what they said was a “view that is very Eurocentric.”
A more full-throated criticism comes from Marianne Franklin, professor of Global Media and Politics at Goldsmiths, University of London, who said that distorted projections like Mercator’s “underpin the ongoing Anglo-Euro-American presumption that the world belongs to them, and pivots around these geo-cultural axes.”
In the case of the Boston public school system, they decided to start purchasing maps based on the Gall-Peters projection, which offers a more accurate depiction of size and area. It has its own drawbacks, though, as it distorts the shapes of continents. Some cartographic historians, like Professor Matthew Edney at the University of Southern Maine, favor the Eckert IV projection, which preserves proportionality without distorting the shape of continents as much as Gall-Peters. The fact is, there are some basically insurmountable obstacles when attempting to perfectly represent a three-dimensional planet in two-dimensional space.
The first known globe is said to have been created in about 150 BCE by a philosopher named Crates of Mallus in Cilicia. The idea of a spherical Earth was considered around 500 BCE by Pythagoras, but if anyone thought to create a globe at the time the evidence has been lost to history.
A century and change later, the Greek philosopher Aristotle was clear that Earth wasn’t flat. He wrote about how moving from north to south meant seeing different stars in the sky, the result of a changing horizon that wouldn’t occur on a flat Earth.
Eratosthenes used observations of how high the sun rose in two different Egyptian cities, along with some relatively simple math, to create a rough estimate of the Earth’s circumference. Despite his rather crude methods, his estimate was in the right ballpark. That was more than 2000 years ago.
The globe, named Erdapfel (which literally means “Earth apple”), is an important piece of history that documents our simultaneously broad and limited understanding of the Earth. It contains myriad errors, including a couple of mythological islands throughout the Atlantic ocean. But give Behaim a break—he didn’t have Google Earth to fact check.
Another of the oldest surviving globes, the Hunt-Lenox globe from around 1510, is also one of the few historical appearances of the phrase HC SVNT DRACONES, or Here be Dragons. The Latin phrase appears below the equator off Asia, which might be a reference to the Komodo dragons in the Indonesian islands.
In 1519, Portuguese explorer Ferdinand Magellan departed Spain with five ships and demonstrated that the globe could be circumnavigated, laying to rest any lingering doubt about its shape. Magellan died during the voyage, and of the five ships only one completed the circumnavigation.
During the 18th and 19th centuries, it was considered somewhat fashionable for middle-class “men of the world” to carry around a pocket globe, a tiny 2-to-3-inch globe that fit right alongside a pocket watch.
On the other end of the spectrum, the largest globe in the world is called Eartha, coming in with a diameter of just over 41 feet. It was developed by the DeLorme mapping corporation and is now housed in an atrium in Yarmouth, Maine. Guinness World Records awarded it the title of Largest Revolving Globe in the world in 1999. Eartha still holds that record today.
While globes can represent the planet in interesting ways, it’s hard for them to compare with the majesty of actual photographs of our planet made possible by advances in technology. On February 14, 1990, NASA’s Voyager 1 spacecraft took a picture of the planet from roughly 4 billion miles away. It depicts the Earth as little more than a speck in the universe. It was taken at the behest of Carl Sagan, who later wrote about it as, “[a] mote of dust suspended in a sunbeam.” He called it the Pale Blue Dot.
Earth has also proven to be quite the muse for musicians. Marvin Gaye’s 1971 hit, “Mercy, Mercy Me” was about ecological conservation. Joe Walsh made an entire album, Songs for a Dying Planet, about it. In 2015, Paul McCartney, Sean Paul, and Natasha Bedingfield, among others, recorded “Love Song to the Earth,” an all-star composition to raise awareness of climate change.
Artists have also taken inspiration from the Earth in more high-tech ways—namely, from Google Earth. Belgian-born artist Mishka Henner uses satellite images as the basis for works like 51 U.S. Military Outposts, which features photos of supposedly low-key military installations.
Not only has Earth been a favorite subject of artists, it’s also been a favorite art material. A conceptual art movement in the 1960s and ’70s brought with it a style known as Land Art or Earth Art, which used soil, rocks, sand, and other humble materials to create pieces. One of its major figures was Robert Smithson, who actually created pieces out in nature like Spiral Jetty, which was made out of earth and basalt. One of the most spectacular earth artworks is Celtic Horse in Slovakia, a 330-foot-by-330-foot geoglyph, otherwise known as a stone sculpture. The artist, Andrew Rogers, has created over 50 geoglyphs, working in 16 countries and all seven continents.
Jules Verne’s Journey to the Center of the Earth, which was published in 1864, illustrates the hazards of descending into the Earth, where the book’s protagonists encounter Jurassic sea monsters.
If we were to make a journey to the core, we’d start at the crust, the outermost layer of the planet. It’s composed mostly of igneous rocks, and contains all life on Earth.
About 4.5 billion years ago, the Earth was a more-or-less uniform ball of hot rock. The Earth is thought to be about 4.54 billion years old, plus or minus about 1 percent. That number is the result of observation, radiometric dating, and a bit of educated guesswork.
Scientists sometimes look for the oldest rocks they can find here on Earth, and analyze the radioactive isotopes within them to determine their ages. Rocks at least 3.5 billion years old have been found on every continent, and zircon crystals in Western Australia can be dated as far back as 4.4 billion years ago. One problem with this method is that plate tectonics can destroy and recycle some of the planet’s oldest rocks, making them less useful for aging the planet.
Helpfully, we have a near neighbor, the moon, that hasn’t been disturbed by plate tectonics, and therefore has many more ancient rocks to collect. Astronauts collected some of these moon rocks back in the ’60s and ’70s, some of which were eventually dated to between 4.4 and 4.5 billion years ago.
Our current best estimate for the Earth’s birthday comes from synthesizing a number of different data points. Radiometric dating of meteorites gives us an idea of when the solar system formed, and analyzing the composition of different isotopes of lead here on Earth helps researchers determine how much time would be required to get to these particular compositions. The resulting estimate—that Earth is 4.54 billion years old—is offered with a relatively high degree of certainty, but questions remain.
We’re not sure how quickly the Earth formed, for example. A study published in February 2020 by researchers at the University of Copenhagen suggested that the Earth formed in about 5 million years, decidedly faster than previous estimates in the tens of millions of years.
The study’s authors suggest that the planet formed through the accretion of cosmic dust, rather than through a series of random collisions between celestial bodies. Five million years might still sound like a lot of time, but if you map the roughly 4.6 billion-year history of the solar system onto a single 24-hour “cosmic day,” then the Earth’s formation, in the researchers’ understanding, took only about one-and-a-half minutes of that cosmic day.
After about 500 million years, the Earth had heated to the melting point of iron, about 2800°F.
This period of superhot temperatures was kind of like the planet’s adolescence, a period of volatility that helped it become the Earth we would eventually know and love. The high temperatures facilitated greater movement of Earth’s rocky, molten material. Buoyant materials like water, silicon, and oxygen tended to float to the surface, forming the planet’s early mantle and crust.
For the first 2 billion years of Earth’s existence, there wasn’t any atmospheric oxygen to take in. At some point, cyanobacteria used the energy from sunlight to make sugar out of water and carbon dioxide, a process known as photosynthesis. Cyanobacteria produced oxygen as a waste product. Science still isn’t sure why cyanobacteria grew from a single-celled to multicellular organism and began to produce oxygen in amounts sufficient to fill the atmosphere, but they did, resulting in what’s been dubbed The Great Oxidation Event.
Without The Great Oxidation Event, we wouldn’t have had the Cambrian Explosion, an evolutionary benchmark that saw a bonanza of life forming, including chordates, which include vertebrates, and many hard-bodied animals like brachiopods that lived in shells. It’s known as an explosion even though it may have taken up to 20 million years.
Fast forward a few hundred million years to today. The Earth’s exterior has cooled and is now divided into two different types: continental crust, which averages between 18 and 30 miles thick, and oceanic crust, which averages 2 to 4.5 miles thick.
Scientists have identified about 1.2 million species of plants and animals, but by some estimates that leaves another 7.5 million species on the planet still to be discovered. Some predict that we have even more work to do, pegging the number of total species on the planet in the neighborhood of 1 to 6 billion. That estimate comes from a group of scientists at the University of Arizona, Tucson, who published a piece in The Quarterly Review of Biology in 2017 which suggested that bacteria could make up 70 to 90 percent of species on earth. When it comes to biodiversity, Earth is still very much an undiscovered territory.
For all that diversity, it’s estimated that humans and farm animals make up an astounding 95 percent of all vertebrate land animals, with wild vertebrate land animals representing just 5 percent of the total.
That’s one reason why a lot of scientists believe we’re facing an environmental crisis—we consume a lot of earth’s natural resources. Between 25 and 40 percent of all energy captured by plants is used by humans and livestock.
Humans might be the dominant species on the planet in terms of resources, but in terms of sheer numbers, over 80 percent of known species—or over 1 million—are insects. If you’ve ever had ants invade your living space, you’re probably not surprised.
Ants, however, don’t bring the numbers when it comes to sheer population density. The Collembolans, or shrimp-like springtails, are an insect relative and a very tiny .25 to 10 mm in size. Roughly 10,000 of them can be found in just a cubic meter of soil. Depending on the location, that number can grow to 200,000. Chances are that the next time you’re standing on solid ground that’s not paved over, you’re standing over a lot of springtails.
If you want variation in species, consider the beetles. Scientists have named over 400,000 species of the insects so far, with more certainly on the way. That means between one in three or one in five described life forms on the planet is a beetle.
Genetic diversity is the different genes in individuals; species diversity speaks to the differences between populations of species and between different species; and ecosystem diversity is the variety of habitats and processes occurring in a given setting.
The next time you pop a prescription or over-the-counter drug to alleviate a headache or other malady, consider raising your glass of water to biodiversity. Most of the drugs developed in the past century were derived from plants, bacteria, and fungi. Willow bark gave us aspirin, and in 2018 there were reports about a potential new antibiotic from the soil of a churchyard in Ireland, which according to the BBC has long been a folk remedy.
Just four crops—wheat, maize, rice, and soybean—are responsible for two-thirds of the world’s food supply. To reduce carbon footprints, researchers are hoping to rely more on the 7000 other crops that humanity has farmed in the past. Maybe one day you’ll be making pancakes with moringa leaves instead of flour.
Colombia is thought to be the most biodiverse country on the planet by area, with one in every 10 species of flora and fauna found there, including more species of birds and orchids than anywhere else. Its many ecosystems enable this diversity. In fact, you could hike from a desert to a tundra in Colombia in just a few days.
They’re the Atlantic, Pacific, Indian, Arctic, and Southern oceans. Most countries now recognize the Southern (or Antarctic) Ocean as a unique, major ocean basin, but it’s still pretty common for some sources to only recognize the first three or four; there are even some who only recognize one “world ocean.”
Either way, the oceans are truly Earth’s treasure … literally. It’s estimated that there are roughly 20 million tons of gold in ocean waters. And not buried in a sunken pirate ship, either—literally in the water. The tiny gold particles, when added all together, would have a value of roughly 1.1 quadrillion dollars. You can’t reasonably sift for it, unfortunately, because one liter of water contains about 13 billionths of a gram of gold.
The Denmark Strait is home to a waterfall below the Atlantic that has a drop of over 11,500 feet. The cold water coming from the east is more dense than the warm water from the west, so when they meet, the cold water drops down, creating a massive waterfall.
This massive mostly-underwater mountain range spans around 10,000 miles. For context, the Andes are about 4300 miles long.
The most remote place in the world is in the Pacific Ocean. Known as Point Nemo, it is the place that is farthest away from any land. The closest islands are over 1600 miles away. It’s so remote that, at certain times, the closest humans to this point are astronauts aboard the International Space Station.
In 1992, over 28,000 rubber duckies were accidentally dumped into the Pacific Ocean. Oceanographers turned lemons into lemonade by tracking where the ducks wound up to better understand the water currents. The duck sightings continued through the mid-2000s.
It landed near its current name in the 16th century, when it went by Oceanus Orientalis Indicus, or Indian Eastern Ocean. It was also known as the Western Ocean by Chinese explorers.
There’s an almost-entirely-submerged continent hiding in the Indian Ocean. The Kerguelen plateau is about 1800 miles to the southwest of Australia and is a result of the Kerguelen volcanic hotspot, which probably formed around 130 million years ago.
The Arctic Ocean is the smallest of the oceans, but that doesn’t mean it’s easy to explore. The first scientific expedition to the central Arctic didn’t occur until the 1890s, thanks to a Norwegian explorer named Fridtjof Nansen who, incidentally, would go on to win the Nobel Peace Prize in 1922 for his work with refugees.
They’re also found in the Atlantic and Pacific. Their tusks seem to serve a mostly social purpose in mating displays and shows of dominance, but they have also been known to use the ivory protuberances to, in the words of the National Oceanic and Atmospheric Administration, “haul their heavy bodies up onto the ice.”
The Southern Ocean is recognized by many governing powers. It is not, however, recognized by the National Geographic Society, which has the Atlantic, Pacific, and Indian oceans extending to Antarctica.
Despite being a pretty cold and uninviting place, the Southern Ocean is home to a bounty of wildlife. Penguins, whales, orcas, seals, and colossal squids call this place home.
Even deep oceans are found on the Earth’s crust, but if we go a bit deeper, we hit the mantle. It’s solid, like the crust, but also contains softer rocks that can move over the course of millions of years. According to National Geographic, “Activity in the mantle drives plate tectonics, contributing to volcanoes, seafloor spreading, earthquakes, and orogeny (mountain-building).”
The temperature in the mantle can range from around 1000°C to 3700°C. As you might expect, it generally gets hotter the deeper you go. There’s even a term for this change: geothermal gradient. Basically, each kilometer you drop in the crust, the average temperature will rise about 25°C (an increase of roughly 1°F for every 70 feet down). It’s slower in the mantle, rising around half a degree Celsius per kilometer, until you get near the core, when the rate of increase picks up again.
You have to descend about 3000 kilometers from the Earth’s surface to end up in the core, about the distance between London and the Norwegian archipelago Svalbard, home to the Svalbard Global Seed Vault—we’re really not that far from our planet’s super-hot core.
Caltech geochemist Paul Asimow told Popular Mechanics, “Within uncertainty, the temperature at the center of the Earth is the same as the temperature at the surface of the sun.” That’s roughly 10,000°F.
Given the temperatures and pressures involved, direct observation of the Earth’s core is impossible. So how do geologists make inferences about it? Dr. Ken Rubin, professor in the department of Earth sciences at the University of Hawaii, addressed precisely that question on the website Ask An Earth Scientist. Dr. Rubin’s explanation provides us with some more cool Earth facts. First, he says, “we know the overall density and mass of the Earth based on measurements of how the Earth perturbs the orbits of other planets and the moon.” Basically, you can’t put the Earth on a scale to get its mass, but you can use our understanding of gravity to come up with a pretty good estimate (with hat tips to Isaac Newton for his law of gravity and Henry Cavendish for determining the “universal gravitational constant”).
The Earth has a mass of roughly 6 sextillion metric tonnes, and its volume is around 1 trillion cubic kilometers.
That sounds pretty big, but it all depends on what you’re comparing it to. Around a million Earths, for example, could fit inside the sun.
Dr. Rubin explains that some other pieces of evidence help tell us about the overall chemical composition of materials on Earth, such as analyzing chondrites. Chondrites are a type of meteorite thought to be essentially unchanged since their formation at the beginning of the solar system. Rubin explains that we can look at information like this to create what is essentially a “balance sheet of materials” of all elements that should be found on Earth, then observe which chemical elements aren’t in the crust or mantle. By process of elimination, geologists reason that these elements must be found in the core.
Additionally, the existence of Earth’s magnetic field tells us there must be a high concentration of metal in Earth’s core—specifically, metal that can remain liquid even under very high pressures. The available evidence and common-sense inferences allow us to assume Earth’s core is primarily iron, with a smaller amount of nickel alongside a lighter element such as oxygen or sulfur.
Here’s an interesting fact about the Earth’s magnetic field: It can flip. If you could stand in the same location, facing the same direction, hundreds of thousands of years apart, a compass would actually tell you you’re facing north at one time and south at another. In fact, geologic evidence suggests that about 170 of these polarity reversals have occurred over the last 76 million years.
A recent study published in the journal Nature Communications suggests that these polarity reversals can happen considerably faster than once thought, though “considerably faster” is obviously a relative term. You’re still talking tens of thousands of years.
Its shape is more accurately—if still imperfectly—described as an oblate spheroid. That just means it’s a bit squashed at the poles and bulges at the center. A variety of forces, from plate tectonics to tides to weird anomalies in gravity, mean the planet isn’t a perfect oblate spheroid. But the phrase does indicate a warping in shape compared to a sphere that gives us a much better picture of reality.
This warping in shape is due to the rotation of the Earth and the centrifugal force it creates. Geologist Vic Baker at the University of Arizona in Tucson compares it to spinning a ball of silly putty, though he notes that “Earth’s plasticity is much, much less than that of the silicone plastic clay so familiar to children.”
The less-than-perfectly-spherical shape of the Earth means that, when standing at sea level, you’re actually closer to the center of the Earth at the poles, compared to the equator. About 21 kilometers closer, in fact.
The greater the distance between the center of mass of two objects, the lower the gravitational force between the objects. Alongside the forces that are bulging the planet, this means that the force of gravity on an object is generally greater at the Earth’s poles than at the equator. So you’d actually weigh a teeny bit less in Ecuador than you would in Antarctica. It’s worth pointing out that your mass would stay the same. We’re referring specifically to weight, which is basically a way of saying the force of gravity on an object.
There are other factors that come into play, though. Higher altitudes bring you farther from the center of the Earth, so your weight decreases a little bit in a high-flying airplane, for example.
The altitude you gain from being on a mountain has a more complicated relationship to gravity. Yes, the altitude brings you farther from the center of the Earth, but the mountain itself provides an additional source of gravity. And the density of the Earth’s crust in a given location plays its own role in your weight, mountaintop or otherwise.
Gravity actually varies across the planet for a number of reasons. One of the biggest variations is observed in Canada’s Hudson Bay region, where the average resident weighs about a tenth of an ounce less than they would in an area with a more typical gravitational force.
GRACE, a joint research project of NASA and the German Aerospace Center, offered an explanation for the relative paucity of gravity in Hudson Bay, identifying two contributing factors. Tens of thousands of years ago, a giant ice sheet covered the area, depressing the Earth’s crust and forcing mass away from the depression. This means that there’s less mass underneath you in Hudson Bay, and therefore less gravitational force. Additionally, convection 100 to 200 kilometers beneath the surface of the Earth likely plays its own role. It’s thought that convection currents drag the continents down, and lower the gravitational force of the area. The GRACE researchers estimated that the impact of the ice sheet could account for 25 to 45 percent of the drop in gravity at Hudson Bay, with the rest attributable to convection currents and tectonic movement.
The constant, if quite slow, movement of tectonic plates has far-ranging effects on the planet. They can give rise to entire islands, like Hawaii, which is actually a series of volcanoes.
Plate tectonics can also cause earthquakes. For instance: when two plates collide, pressure builds up. When the plates eventually break loose, the pent-up pressure can result in tremors that are undetectable without instrumentation or in a massive natural disaster.
The largest earthquake ever recorded with reliable instrumentation is 1960’s Valdivia earthquake in southern Chile. It was measured as a 9.5 on the moment magnitude scale. The moment magnitude scale is logarithmic, meaning the massive Chilean quake released almost 8000 times more energy than San Francisco’s devastating 1989 earthquake, which registered a 6.9.
Think of weather as a single data point, telling you about a moment in time, and climate as a collection of data points. Climate is the trend of weather patterns. Spoiler alert for planet Earth: The trends aren’t great.
Earth’s weather phenomena, though, can be truly fascinating. In 1991, a tornado carried a cancelled personal check from Stockton, Kansas, to Winnetoon, Nebraska—a distance of about 223 miles.
It’s not a well understood process, but science historians believe that mentions of phenomena that sound like ball lightning emerged as early as 1557.
From ancient Greek mythology to modern religious prayers, summoning specific weather patterns has been a recurring goal for some human beings. For example, the Berwick witches of Scotland attempted to use black magic to sink the ship of King James VI of Scotland by summoning storms. Or, at least that’s what they were accused of.
There are some modern attempts to control the weather, the most common being cloud seeding, a process that tries to change precipitation in clouds. This is done either to promote rainfall or to suppress fog at airports, for instance. Seeding clouds often uses chemicals such as silver iodide or dry ice, but its effectiveness is debatable.
Unfortunately, modern attempts to modify the weather have been used for dark purposes as well. For example, Operation Popeye was an American military tactic to promote rainfall in Vietnam. The hope was that cloud seeding would prolong the monsoon season and therefore disrupt supply lines, and though the operation was carried out, as Popular Science notes, “Its stated objective—to ensure Americans won the Vietnam War—was never realized.” Since then, “weather warfare” has been banned by the UN under the Environmental Modification Convention.
Earth’s atmosphere is a complex and beautifully layered shield from the nightmare that is outer space. The troposphere is the lowest layer of Earth’s atmosphere, extending from the surface to, on average, about 12 kilometers above the surface. Although it is the shortest layer, it contains about 80 percent of the mass of the atmosphere.
The stratosphere extends to about 50 kilometers above the surface, and it’s home to the ozone layer.
While “ozone” might sound like a Power Rangers villain, it’s actually just trioxygen, or O₃, a molecule of oxygen with three atoms instead of two. Ozone has been described as smelling like chlorine, a burning wire, or an electrical spark.
The top of the mesosphere is known as the mesopause, which is the coldest natural place on Earth. The average temperature is about -120°F (-85°C).
Next up is the thermosphere, which has an altitude range of anywhere from 500 to 1000 kilometers. Changes in solar activity can affect this altitude greatly. It’s also home to the International Space Station.
And finally, we have the exosphere. This layer extends to about 10,000 kilometers above sea level, though some peg that number as high as 190,000 kilometers, or about half the distance to the moon—after all, it’s kind of hard to say where Earth ends and “space” begins. The density of molecules is so low in the exosphere that atoms can travel hundreds of miles before colliding with one another.