Archimedes' principle says an object is buoyed up by the weight of the fluid it displaces. A floating object settles until the displaced fluid weighs exactly as much as the object. If the object is less dense than the fluid — meaning the fluid it would displace when fully submerged weighs more than the object — it floats with only part of itself underwater.

A wooden block displaces water too, but wood is less dense than water. Before the block is fully submerged, the buoyant force already equals the block's weight. It floats with part above water, part below — the exact ratio matching the density difference.