Animals that undergo metamorphosis are insects and some types of amphibians. Metamorphosis can be complete, which involves four stages including a pupal stage, or incomplete, wherein the pupal stage is avoided.
Complete metamorphosis occurs in the butterfly. The butterfly begins as an egg that hatches into a caterpillar. The caterpillar does not resemble its parent, but looks like a worm. It has chewing mouthparts that help it devour the great quantities of food it needs to grow.
The caterpillar molts several times as it grows and finally enters a pupal stage. Here, it shuts itself up in a chrysalis or cocoon. Inside the cocoon, drastic changes take place to change the caterpillar into its adult form. Though the pupal stage may last about a fortnight, some pupae overwinter if they pupate in the fall. Then, the adult butterfly emerges from the cocoon and flies off.
Though it's also an insect, a grasshopper undergoes incomplete metamorphosis. The grasshopper nymph hatches out into what looks like a miniature adult and simply molts until it reaches adult size. A frog also begins its life as an egg, but most frogs are tadpoles that live in water. Over time, they grow legs and lungs, and their tails are absorbed into the body. Eventually, they become a terrestrial frog.