I would add another curious fact about buoyancy. Inside an accelerating car, a helium balloon will move toward the front of the car, because the acceleration forward pushes the air toward the rear of the car, making it more dense on that side of the balloon. So the balloon "floats" on top of that denser air mass. Likewise, if you hit the brakes, the balloon moves to the back of the car, because now the air in the front is denser.

Incidentally, the similar effects of gravity and acceleration are what led Einstein to develop the General Theory of Relativity.