If a cup of water spills on the floor, the water can’t unspill—that is, it’s inconceivable that each water molecule would exactly reverse its course to slip back into the cup. To do so would be to ...

During a solar eclipse, astronomers who study heliophysics are able to study the sun's corona—its outer atmosphere—in ways they are unable to do at any other time. The brightest part of the sun is so ...