Ask an Explainer

For something to float in the air it has to be less dense than air - or lighter than air.

The best example of this is a balloon. Balloons full of helium, or hot air balloons, both are basically a bubble of less dense air. Since it is less dense than the atmosphere, the air around the balloon will push them up, until they reach a place where their inside density meets the outside density. 

Kites and parchutes glide on the air. They don't float: they're much too heavy for that. They use the forces of drag and lift to stay in the air as long as possible.