Q: What is considered the world’s first currency?

— Dennis Macmath | Waterford, Michigan

It’s a bit tricky, says Ellen Feingold, curator of the National Numismatic Collection at the National Museum of American History, because we don’t know what the first objects used as a medium of exchange were. But the earliest coins were used around the sixth or seventh century B.C., and they appeared in China and in Lydia, an Iron Age kingdom in western Anatolia. The Chinese cast copper-alloy coins in the shape of farming tools. The Lydian coins were made of electrum, a mixture of silver and gold, and were struck with an image of a roaring lion. Paper money wasn’t introduced until the 11th century A.D., in China.

Q: Are there any mammals with green hair or fur? It would seem to be effective camouflage in forests and fields.

— Brian Gerber | Pine Plains, New York

There aren’t. Precisely why isn’t certain, but it probably comes down to the hair pigment, says Steven Sarro, a supervisory biologist and curator at the National Zoo. Mammalian hair has two kinds of pigments: one produces black or brown hair and the other yellow or reddish orange, but no mixture of the two would ever result in green. And yet it’s not impossible that a mammal could appear to be that color, thanks to environmental influences. For instance, sloths hanging out in wet rainforests frequently have green algae growing on their fur, and polar bears have hollow guard hairs, which form their outer fur and in which algae often grow, especially when the bears are in captivity.

Q: What is the meaning of the term “D-Day”?

— Jack Buege | Belmont, Michigan

While the expression is indelibly linked with the Allied invasion of Normandy on June 6, 1944, it actually comes from U.S. Army terminology that was first used in World War I and is still used today, says Frank Blazich, curator of modern military history at the National Museum of American History. Generally, “D” refers to the first day of an operation or attack; it serves as a place holder in the planning phase, and even after an exact time is set, soldiers continue to use the term to prevent unauthorized disclosure. “D-2” and “D+2” refer to two days before and two days after an action is scheduled to begin. “H” refers to hours, as in “H+7,” or seven hours into an operation.

Q: Do your ears pop when you’re traveling into space?

— Beth Lamond | Driffield, Alabama

No, says Janet Kavandi, a veteran of three spaceflights and a recent inductee into the United States Astronaut Hall of Fame, at the Kennedy Space Center Visitor Complex, a Smithsonian affiliate. Before liftoff, the space shuttle is pressurized to the normal atmospheric pressure, and because the shuttle is airtight, the pressure doesn’t decrease during ascent, as it does in an airplane. Typically, the pressure aboard the shuttle remained constant and equivalent to the atmospheric pressure at sea level on Earth unless it was deliberately reduced to help crew members prepare for a spacewalk. That pressure reduction was only the beginning of an elaborate protocol to prevent astronauts from suffering from decompression sickness, or “the bends,” while outside the spacecraft, because the pressure in a spacesuit is about a third of that in the cabin.

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