This S-shape movement, also known as undulatory locomotion, is used by most snakes on land and in water. Starting at the neck, a snake contracts its muscles, thrusting its body from side to side, creating a series of curves. In water, this motion easily propels a snake forward because each contraction pushes against the water. On land, a snake usually finds resistance points in the surface -- such as rocks, branches or dents -- and uses its scales to push on the points all at once, thrusting the snake forward. Speckled rattlesnake using rocks as resistance points