Helium balloons slowly shrink because the atoms are small enough to escape through the rubber.

The longer version

Rubber looks solid but is a tangle of long molecules with gaps between them. Helium atoms are the smallest of all, small enough to slip through spaces that hold back larger gas molecules. Over hours they leak out through the wall, and the balloon slowly loses volume.

The same leakage happens with any gas, just far more slowly, and thinner or stretched rubber lets atoms through more easily. Air-filled balloons contain mostly larger nitrogen and oxygen molecules, so they hold their shape much longer. Eventually the lift disappears as the gas inside gives way to ordinary air.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.