What do caterpillars go into to become a butterfly?

It's a process called metamorphosis - triggered by genes in the caterpillars body. After a caterpillar hatches from an egg, it basically spends all its time eating - growing and preparing its body for the change to a butterfly. During the caterpillar stage, it moults its skin three times - growing larger each time. Eventually, it spins a silk cocoon, and the transformation takes place over a number of days. While hidden in the cocoon, the caterpillars body is literally re-built - growing legs, wings, and flight muscles.