American paper money is printed in just two places — at the Bureau of Engraving and Printing’s facilities in Washington, D.C., and Fort Worth, Texas — but once it’s entered circulation, it ends up all around the world. The greenback is so well received worldwide that U.S. dollars are commonly accepted in other countries, not just at banks but in everyday interactions. However, most of the U.S. money that leaves the country never returns, and financial experts believe anywhere from half to two-thirds of all U.S. circulating banknotes are actually kept outside of the country’s borders. 

All U.S. banknotes weigh the same amount.

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Regardless of their face value, all seven denominations of U.S. paper money — $1, $2, $5, $10, $20, $50, and $100 — weigh the same amount. When placed on a scale, each bill measures just 1 gram. It would take 454 banknotes to equal 1 pound.

It’s difficult to track down exactly where American currency winds up after it leaves the country. While the U.S. Customs Service requires travelers and others to report incoming and outgoing cash, and banks monitor large shipments of bills as they enter and exit the country, there isn’t any other way to track how U.S. bills are used (or where they end up) after crossing American borders.

Economists believe U.S. money has become popular outside of the country thanks to its stability. Citizens in countries with less reliable currencies often practice “dollarization”: using U.S. dollars alongside (or as) their homeland’s own form of tender. Swapping to a more secure form of money that better retains its value protects from tumultuous ups and downs that can cause financial losses. While greenbacks are commonly accepted all throughout the world, at least 11 countries have adopted U.S. dollars as their official currency, including Ecuador, Zimbabwe, Micronesia, and the British Virgin Islands.

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Few things are as ubiquitous as the metric system, also known as the International System of Units. Democratic societies, totalitarian regimes, desert nations, and mountainous countries alike all use the decimal system of measurement first devised in the 18th century — all, that is, except Liberia, Myanmar, and the United States. Instead, these countries use imperial (or customary) measurements, such as the foot, yard, and mile, as well as ounces, pounds, and tons. Even among these three nations, Liberia and Myanmar are in the middle of the metrification process, meaning that one day the U.S. will be the lone holdout. 

“Smidgen” is a real measurement.

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“Dash,” “pinch,” and “smidgen” are often used in recipes, roughly translating to “a little.” But these terms actually have real measurements associated with them. A dash is 1/8 teaspoon, a pinch is 1/16 teaspoon, and a smidgen (also known as a shake) is 1/32 teaspoon.

There was a time when the U.S. was also headed in the metric direction. In 1975, Congress passed the Metric Conversion Act, calling for the country’s voluntary conversion to the metric system — the key word there being “voluntary.” President Jimmy Carter tried to implement the transformation, but the effort fell flat during Ronald Reagan’s subsequent administration. Because of the system’s near-ubiquity, however, today some U.S. groups (especially scientists) and businesses choose to use metric measurements anyway. And while many other countries have moved on from their own traditional measurement units, some still retain vestiges of time past. In England, road signs still show miles, while some Asian countries sometimes still use traditional measurements in unofficial capacities. And although the U.S. is unlikely to ditch its customary units any time soon, the country is more metric than meets the eye — after all, we often buy beverages in liters and run 5Ks, even if our recipes call for cups and spoons and our odometers measure in miles.

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Earth is home to stunning snow-capped mountains that tower over their surrounding landscapes, but none quite compare to Mars’ Olympus Mons. First photographed in detail by NASA’s Mariner 9 probe in 1971, Olympus Mons (Latin for “Mount Olympus”) is nearly 16 miles tall. For comparison, its most famous Earthly competitor — Mount Everest — is only about 5.5 miles above sea level. The width of Olympus Mons is just as impressive as its height: Stretching 374 miles across, it’s as big as the entire state of Arizona. Olympus Mons is what’s known as a shield volcano — a type formed as lava slowly spreads out and cools; they usually have a low profile and are named for their resemblance to a warrior’s shield.

Earth is the most volcanically active place in the solar system.

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Io, a moon of Jupiter, is the solar system’s most volcanically active spot, with hundreds of volcanoes erupting every moment. The tiny moon is influenced by the gravity of Jupiter and sibling moons, which create tides and friction that heat it and cause major volcanic activity.

So how did Olympus Mons get so big? Scientists think a combination of low surface gravity and high volcanic activity allowed Mars’ great shield volcano to grow — over billions of years — beyond anything seen on Earth. And unlike on Earth, where volcanoes form as tectonic plates drift over hot spots of lava, Mars’ plate movement is much more limited, meaning magma can build and build in one spot over a long time. So while summiting peaks like Everest and K2 remains an impressive terrestrial feat, the solar system’s biggest climbing challenge awaits on the red planet.

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At the start of the 20th century, before the Wright Brothers finally got their famous Flyer off the ground in 1903, airships were seen as the future of human flight. The category includes a variety of dirigibles, such as zeppelins (which have a rigid structure) and blimps (which completely collapse when deflated). German zeppelins performed bombing runs in World War I, but the 1937 Hindenburg disaster — in which the Hindenburg zeppelin caught fire in New Jersey while attempting to moor, killing 36 — spelled the end of airships as commercial vehicles. While blimps found limited use during World War II, after the war, airships mostly transformed into floating advertisements.

The first nonstop transatlantic flight was by airship.

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Airplanes barely beat airships across the Atlantic. In June 1919, English aviators John Alcock and Arthur Whitten Brown flew a twin-engine biplane across the ocean. Weeks later, the British airship R-34 landed in Long Island, New York, after a 108-hour transatlantic flight.

Today, only about 25 blimps exist, and about half of them are used for advertising — including the famous Goodyear Blimp (which is now technically a zeppelin). Airships are expensive to construct and to run, in part because they require as much as $100,000 worth of helium per trip, and helium gas is the subject of frequent worldwide shortages. There’s also a dearth of people trained to fly them. Yet efforts have been made to resurrect the airship, chief among them the U.S. Army’s Long Endurance Multi-Intelligence Vehicle, which was designed to float above the battlefield, providing intelligence, surveillance, and reconnaissance for ground forces (the project was scrapped in 2013). Today, some experts call for a return to airships, but for now they remain part of a lighter-than-air future that never was.

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The galaxy we call home is unfathomably enormous. With enough room for an estimated 100 billion planets, the Milky Way stretches about 100,000 light-years across, although estimates of its full size vary. (A light-year is the distance a beam of light travels in one year on Earth, equal to about 6 trillion miles.) Earth is situated approximately two-thirds of the way from our galaxy’s center; we’re essentially in the suburbia of the Milky Way. When we look at celestial bodies, we’re actually looking back in time, because of how far away they are and how long their light takes to reach us. The sun we see, for example, is always about 8.3 minutes old, while the light from the North Star, aka Polaris, is about 320 years old. And while we can’t actually see the center of the Milky Way, light from the area takes nearly 25,000 years to reach our planet. That means it dates back to when humans were still in the Stone Age. 

The inventor of the Milky Way candy bar was inspired by space.

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Mars launched the Milky Way bar in 1923 under the same name as a popular malted milkshake flavor; the galaxy was not involved. Ads claimed the candy was healthier than other treats thanks to its malted milk powder, helping Mars to sell more than 16 million bars in the product’s first year.

What we know about our galaxy is ever-expanding — much like the universe itself. Early astronomy pioneers such as Aristotle believed the Earth was the center of the universe, circled by the sun, moon, and all other cosmic matter. In 1609, Galileo’s first glimpse of the Milky Way through an improvised telescope showed its wispy appearance wasn’t a layer of clouds, as previously thought, but a vast collection of individual stars. His discoveries lent credence to the idea that Earth wasn’t the center of the universe after all. Yet it would take 300 more years for scientists to confirm that we’re not even at the center of our own galaxy — it wasn’t until 1924 that astronomer Edwin Hubble confirmed that the Milky Way is just one of many galaxies in our vast universe. 

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