concept

Archimedes' principle

A body immersed in a fluid experiences an upward force equal to the weight of the fluid it displaces. The point most often missed: buoyancy does not depend on how light the object is, but on how much it pushes aside. A stone feels that upward force too — it simply weighs more than the water it displaced, so it sinks. Because air is also a fluid with weight, the principle governs the sky exactly as it governs the sea, and it is the entire basis of the balloon, the airship, and any organism that chooses to hang in the air rather than beat against it.