Hundreds of years ago, Galileo theorised that objects should fall at the same rate regardless of their mass when there is no air resistance.
This hypothesis must have seemed confusing at the time, but in 1971 he was proven correct when an astronaut but the theory to the test on the moon.
Check it out here:
On his last day on the moon, Apollo 15 Commander David Scott conducted a little experiment using a hammer, weighing 1.32kg, and a feather, weighing 0.03kg (an experiment he presumably planned in advance, 'cause I'm not sure where he'd have found a hammer and a feather lying about on the moon).
As more than 600 million people back on Earth watched on live, Scott held aloft the objects and said: "In my left hand I have a feather, in my right hand a hammer.
"And I guess one of the reasons we got here today was because of a gentleman named Galileo, a long time ago, who made a rather significant discovery about falling objects and gravity fields.
"And we thought where would be a better place to confirm his findings than on the moon? So we thought we'd try it here for you.
"The feather happens to be, appropriately, a falcon feather for our Falcon [spacecraft], and I'll drop the two of them here and hopefully, they'll hit the ground at the same time."
Sure enough, the feather and the hammer fell at the same speed, hitting the moon at the same moment.
"How 'bout that - Mr Galileo was correct in his findings."
On Earth, the feather would of course hit the ground after the hammer.
That's because we have good old delicious air, which causes friction with objects as they fall through it, and this air resistance differs depending on the object.
Joe Allen, who was part of the astronaut support crew, wrote in a report: "During the final minutes of the third extravehicular activity, a short demonstration experiment was conducted.
"A heavy object (a 1.32-kg aluminum geological hammer) and a light object (a 0.03-kg falcon feather) were released simultaneously from approximately the same height (approximately 1.6 m) and were allowed to fall to the surface.
"Within the accuracy of the simultaneous release, the objects were observed to undergo the same acceleration and strike the lunar surface simultaneously, which was a result predicted by well-established theory, but a result nonetheless reassuring considering both the number of viewers that witnessed the experiment and the fact that the homeward journey was based critically on the validity of the particular theory being tested."