Why Does a Balloon Stay Blown Up?

Step 1 of 5

Air is molecules flying about.

Air is not nothing. It is countless molecules moving at hundreds of metres per second, colliding with everything they meet.

Step 2 of 5

Their collisions are what pressure is.

Each impact on the rubber is a tiny push. Billions of them every instant add up to a steady outward force on the whole surface.

Step 3 of 5

Squeezing them in raises the pressure.

Blowing forces more molecules into the same space, so there are more collisions per second and the push gets stronger.

Step 4 of 5

Stretched rubber pulls back just as hard.

The rubber's tension grows as it stretches, until it exactly matches the air's push. At that point the balloon stops growing.

Step 5 of 5

It slowly shrinks anyway.

Air molecules squeeze through gaps in the rubber over hours. Helium atoms are smaller and escape faster, which is why helium balloons droop first.

Let go of an untied balloon and escaping air pushes it forward. That is exactly how a rocket works.