Performing Galileo's Thought Experiments on the Moon
Galileo once described a thought experiment where he dropped two different weights off the Tower of Pisa to see which would hit the ground first. This thought experiment argued against different weights falling at different rates, instead arguing all such differences were due to an external force (air resistance)
Apollo 15 astronaut David Scott took advantage of the lack of an atmosphere on the moon to actually perform Galileo's famous thought experiment using a hammer and a feather. A YouTube video of this momentous event is embedded on this page.