What’s the difference between moths and butterflies, in evolutionary terms?

— David Hayes | Baltimore, Maryland

Night and day—literally. Most moths are nocturnal; butterflies are essentially moths that have evolved to be diurnal, or active during the day, says Robert Robbins, a curator of lepidoptera at the National Museum of Natural History. Both moths and butterflies have patterned wings, but the latter also developed brightly colored wings. These colors are codes—to other butterflies, they might signal sex and mating status; to predators, they might imitate a toxic or foul-tasting species. Many moths communicate differently. For example, they may supplement their wing patterns with fragrances to attract mates or repel predators.

Did Thomas Edison really try to develop large-scale affordable housing? What made his design special?

— Paul Lalonde | Guelph, Ontario

He did, with a house of concrete. In the 1890s, Edison developed rock-crushing machinery for retrieving iron ore. That business failed, but the machinery proved well suited to producing cement, a key ingredient of concrete. In 1907, a time when New York City was rife with overcrowded tenements, Edison announced plans for low-cost, healthful concrete houses, and he later patented a method for building them. In a single pour into an iron mold, explains Joyce Bedi, senior historian at the Lemelson Center for the Study of Invention and Innovation, concrete would form the walls and roof—and the stairs, floors, even bathtubs. Edison offered the use of that patent free, and an investor built prototypes in New Jersey, some of which still stand. But this business also failed: The molds were costly, the houses weren’t very attractive, and potential buyers were put off by the stigma of a home labeled “the salvation of the unwashed masses.”

Why are the planets and moons in our solar system spherical?

— Stephen Cohen | Bethesda, Maryland

Objects the size of planets, and some moons, have enough gravitational potential energy to draw whatever they’re made of—solids, liquids or gases and vapors—toward their centers, resulting in a sphere, says David DeVorkin, senior curator at the National Air and Space Museum’s division of space history. That gravitational pull is one of the traits of a planet, per the International Astronomical Union. Some planets, such as Jupiter and Saturn, are less than perfectly spherical because the force of their rotation distorts their shape. Most large moons are also spherical, but they don’t need to be by definition; Mars’ Deimos and Phobos are two slightly misshapen examples.

Why were Lincoln, Jefferson, Washington and Theodore Roosevelt chosen for Mount Rushmore?

— Jacob Guiton | Overlook, Pennsylvania

That was the doing of Gutzon Borglum, the sculptor recruited in 1924 to create “a heroic sculpture” to spur South Dakota tourism. He wanted the Rushmore commission “to communicate the founding, expansion, preservation and unification of the United States.” So: Washington (founding), Jefferson (expansion) and Lincoln (preservation and unification). Roosevelt, says James Barber, historian and curator at the National Portrait Gallery, was chosen to represent the nation’s development and to carry the monument’s narrative into the 20th century.

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Anna Diamond| READ MORE

Anna Diamond is the former assistant editor for Smithsonian magazine.