A balloon is a flexible bag that expands when filled with air or a lighter-than-air gas like helium. The rubber or plastic stretches as you blow into it, and once tied off, it holds its shape until the air escapes or the material breaks.

Party balloons filled with regular air eventually sink to the ground, but helium balloons float upward because helium weighs less than the air around it. If you let go of a helium balloon outdoors, it rises higher and higher until it finally pops or deflates.

Hot air balloons work on a similar principle but on a much larger scale. These enormous balloons carry baskets underneath where people can ride. A burner heats the air inside the balloon, making it less dense than the cool air outside, which causes the whole contraption to lift off the ground. Pilots control altitude by heating or cooling the air inside.

The word can also be a verb meaning to swell up suddenly, like when expenses balloon out of control or when a small problem balloons into something much bigger. In cartoons, speech bubbles are sometimes called balloons because of their rounded, inflated shape.