How does a wriggly water tadpole become a hopping land frog?

A tadpole is basically a swimming blob with a tail: no legs, no jaws for biting, breathing underwater like a fish. A few months later that very same animal is a frog, hopping about on land and snapping up flies. It did not grow the slow, steady way you do, keeping the same shape all along. It rebuilt itself almost from scratch. How?