Caterpillars hang upside down from a twig or branch and spin a cocoon around their bodies. Their bodies transform while in the cocoon, and when they they are totally different insects with wings.

After this transformation, they become butterflies.

Explanation:

This is a natural process of how butterflies develop from caterpillars. It is also usually used a form of figurative speech, where the development from a caterpillar into a butterfly can be used as a comparison to personal growth or broadening one's mind.