We all know it. Gallileo’s Leaning tower of Pisa experiment.
If you drop a brick and a feather at the same time the brick will probably hit the ground first. But this is because of differences in the amount of friction between these objects and the air around them, not because their masses are different. If there were no air, the feather and the brick would hit the ground at the same time.
But actually seeing it really is a wonderful thing.
Real test carried out in the biggest vacuum chamber