There is something incredibly neat about helium balloons! If you buy one at the circus or fair, you can hold its string and it will ride along above you. If you let go of the string, it will fly away until you can't see it anymore.
If you have ever wondered why it flies away, then read on.
Most of us feel comfortable with the idea of something floating in water. We see that happen every day. In fact, people themselves float in water, so we have a way of directly experiencing water flotation. The reason why things float in water applies to air as well.
Helium balloons work by the same law of buoyancy. In this case, the helium balloon that you hold by a string is floating in a "pool" of air.