The equivalence principle is one of the fundamental concepts behind gravitational theory and general relativity, and holds that gravity exerts the same force on objects of different mass.
Around the turn of the 16th Century, Galileo famously demonstrated this principle experimentally by noting that two heavy balls of different mass, when dropped at the same time from the same height, will reach the ground simultaneously.
Of course, if you try to reproduce Galileo's experiment using, say, a bowling ball and a leaf, you're liable to wind up with a different result. The reason for this, as we all know, is air resistance—but not many of us have actually had the opportunity to observe empirical evidence behind this explanation.
But as you might imagine, Earth's Moon is free of air resistance. So in 1971, Apollo 15 astronaut David Scott took advantage of the opportunity to drop a hammer and feather towards Moon's surface (our ability to put a man on the Moon must not have been enough proof that we understand how gravity works). So go ahead, see for yourself what happened.
[Spotted on NASA's APOD]