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you can't have everything fall down because there isn't enough room there. so instead gravity makes it so that heavier things are on bottom and lighter things on top. which is why lighter-than-air things such as baloons and clouds can float
to understand helicopters you need newton's third law and that air also has a mass; hence, pushing air down with the rotor makes the rotor go up and pull the helcopter with it
to understand airplanes you need to understand the aerodynamics of a wing.
