How do tadpoles turn into frogs?

A frog's life begins as an egg laid in water, which hatches into a tadpole that breathes with gills, swims with a tail and usually feeds on algae and plants. As it grows, rising levels of thyroid hormone switch on metamorphosis. The back legs appear first, followed by the front legs, while lungs develop and the gills disappear. The mouth widens and the gut shortens as the animal prepares for a diet of insects, and the tail is gradually absorbed into the body, providing energy for the change. In the common frog this takes around three to four months, but American bullfrog tadpoles can take a year or more to transform.