Q: Why are so many South American animals smaller than their counterparts on other continents?

—Donald Dorfman | Sacramento, California

Smaller animals have an easier time navigating dense rainforests, says Melissa Hawkins, curator of mammals at the National Museum of Natural History. Deer in the Amazon, for instance, evolved to have narrower antlers than North American deer. Human activity can also influence animal size. The African elephant evolved to roam wide-open savannas, but trophy hunters selectively killed off the largest members of the species. You’d be hard-pressed nowadays to find one as large as Henry , the 13-foot-tall elephant on display at the Natural History Museum.

Q: How did NASA decide which astronaut got to step on the Moon first?

—David Miller | Tulsa, Oklahoma

It was part luck, part merit, says Teasel Muir-Harmony, curator of the Apollo collection at the National Air and Space Museum. During the Apollo missions, different groups of astronauts rotated through the schedule, and backup crews became primary crews three missions later. Neil Armstrong was originally scheduled for a later mission, but delays in developing the lunar module shifted him to Apollo 11. Based on Armstrong’s performance in the earlier Gemini missions, NASA made him the commander of Apollo 11, positioning him to step on the Moon first and take that giant leap for mankind.

Q: Why do different types of quarks have such whimsical, nonscientific-sounding names like “strange” and “charm”?

—Mai Cwajbaum | San Jose, California

In some cases, they’re aptly named, says Hossein Sadeghpour, senior scientist at the Smithsonian Astrophysical Observatory. These basic components of nuclear particles like protons and neutrons come in what scientists call six “flavors”: up, down, top, bottom, charm and strange. When a particle in one experiment lasted longer than expected, scientists named the quark involved in the process “strange.” “Charm” quarks were named before they were even discovered, perhaps because their predicted behavior would make the scientists’ theories work like a charm. Top and bottom quarks used to have whimsical names, too—“truth” and “beauty.” The word “quark” itself was inspired by a nonsense line from James Joyce’s Finnegans Wake : “Three quarks for Muster Mark.’’

Q: When and how did stamps start featuring famous Americans who weren’t politicians?

—John David Pratt | Glens Falls, New York

The first U.S. stamps, released in 1847, featured George Washington, the first president, and Benjamin Franklin, the first postmaster general. Designs gradually broadened, but it wasn’t until the Famous Americans series of 1940 that artists, scientists and writers appeared in significant numbers, says Daniel Piazza, curator of philately at the National Postal Museum. In 1957, to keep up with requests, the post office formed a Citizens’ Stamp Advisory Committee; composed of 10 to 15 citizens, it still exists today. Over 800 people have now been featured on U.S. stamps, from Sacagawea to Andy Warhol. The living aren’t officially allowed on stamps, but there have been a few exceptions—including Neil Armstrong, presumably the man inside the spacesuit on the “First Man on the Moon” stamp that came out in 1969.

It’s your turn to Ask Smithsonian .