To be fair, that is not a proof of gravity though. It is only a proof that things heavier than air fall to the ground.
I don't absolutely know if gravity exists, since I can't see it, taste it, smell it, etc.
I do know that a 20 oz Estwing hammer fell to the ground and I have to climb all the way down the ladder to retrieve it. Hammers are heavier than air.
If I farted while on a ladder and the fumes were lighter than air and wafted to the heavens (truly a heavenly fart!!) it wouldn't be a proof gravity DOESN'T exist.
On second thought, if I farted methane and it was heavier than air, not because it had chunks of feces in it, which technically would be a shart and not a fart, but a real live rip roaring methane fart and it fell to the earth, it would still only be a proof that methane is heavier than air.
Point being, I can see some things which are heavier than air fall to the earth, but I can also see or demonstrate some things which are lighter than air rise AWAY from the earth.
I don't absolutely know if gravity exists or not, I do know we've been told it does and many if not most accept that on face value. Also, I think I might need to change my underwear. Oh shit!