Why do some things float and other things sink? It depends on the density of the object. An object is made of substances that are themselves made up of molecules. Density is a measure of both how massive those molecules are, and how closely packed together they are. If the object's total density is greater than that of water, it sinks, but if it is less, it floats.

How do ships float?

Boats and ships—especially enormous steel-hulled ships—look too heavy to float. But because the ship is hollow it contains a lot of air. It is this air that, makes the ship, overall, less dense than water, so it floats. The force of the water pushing upwards against something is called the upthrust, or bouyancy. When something floats, the upthrust exactly matches the force of gravity, which pulls the object down.

Floating in air

Floating in air is the same as floating in water. Anything that is less dense than air will float in it. When you heat air it becomes less dense because its molecules spread out. Hot-air balloons use this principle to fly. Air inside the balloon is heated, which makes it less dense than the cooler air outside. This causes the balloon to rise up into the air.