Q: Why can’t we use energy from volcanoes like the one at Yellowstone to generate electricity? Iceland is almost totally powered by volcanoes.

— Jamie Sorensen | Denver

It’s certainly a tempting idea. Yellowstone has the largest concentration of geysers on earth and scientists estimate that it has enough geothermal energy to power the whole country. But it’s not possible with today’s technology to harness and distribute all that energy, says Liz Cottrell, a geologist at the National Museum of Natural History. And since 1970, the park, a Unesco World Heritage site, has been legally protected from potentially damaging geothermal plants. When New Zealand built a geothermal plant near Wairakei Basin in the 1950s, it ended up destroying the area’s 70 geysers and 240 of its hot springs. Iceland avoided this fate because its hot fluids are so close to the surface, and because the entire country rebuilt its infrastructure, starting in the 1970s, with geothermal energy in mind.

Q: How did Albert Einstein contribute to the Manhattan Project?

— Anonymous | Beaufort, South Carolina

Mostly with his signature, says Roger Sherman, associate curator of the modern physics collection at the National Museum of American History. In 1939, Einstein signed a letter, prepared along with the physicist Leo Szilard and addressed to President Roosevelt, that discussed uranium fission and the possible use of atomic bombs in war. Though Einstein later called the letter his greatest mistake, he also said, “There was some justification—the danger that the Germans would make them.” His primary contribution to the war effort was studying and developing underwater weapons for the U.S. Navy’s Bureau of Ordnance.

Q: Do tigers and other large felines purr like domestic cats?

— Rob Loughridge | Honolulu

Most big cats can’t purr, explains Craig Saffoe, curator of the Great Cats area at the National Zoo. In lions, tigers and other such felines, the hyoid apparatus, which supports the larynx and tongue, is mostly cartilage. When air pushes through the throat, the vibrations come out as booming vocalizations, like bellows or roars. In domestic cats, the hyoid apparatus is mostly bone, which leads to softer, purring vibrations. Some smaller wild cats share this trait, which means cheetahs and cougars can purr.

Q: How and when did scientists figure out space was a vacuum?

— Brooke C. Stoddard | Alexandria, Virginia

It only took a couple of millennia, jokes David DeVorkin, senior curator at the National Air and Space Museum’s Space History Division. The pre-Socratic Greek philosophers first theorized the concept of a vacuum. In the 17th century, Blaise Pascal and other scientists experimented with barometers and saw that pressure diminishes as altitude climbs. Starting in the 1940s, ballooning and rocketry became more sophisticated, and scientists observed that the lower the satellite, the more likely it was to experience atmospheric drag. In the 1964 Echo 2 project, NASA sent a mostly flattened balloon with very little gas into space. When the balloon was released from the satellite capsule, it blew up to 135 feet in diameter, signaling incredibly low pressure. There’s no such thing as a perfect vacuum, but space is a close approximation.

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