Why do balloons float up?
For grown-ups: the science
A helium balloon floats up because it is lighter than the same-size amount of air around it.
Helium has a much lower density than air. A helium-filled balloon displaces air, producing a buoyant force; it rises when this upward force exceeds the weight of the balloon, helium, and any attached string.
What about...?
Ask your child's own question
Transcript
A bright balloon slips from a hand and rises into the sky. Why does it float up?
Air is all around us, even though we cannot see it. Air has weight and pushes upward on things in it.
A balloon filled with helium has lighter gas inside than the air outside. The surrounding air pushes it up more than gravity pulls it down.
The balloon rises until it reaches thinner air, or until it bumps into something. That is why helium balloons can float overhead.
So balloons float up because the air around them gives them an upward push.