Web-footed means having toes connected by thin flaps of skin called webbing. Ducks, geese, frogs, and some otters are web-footed animals. If you've ever watched a duck glide across a pond, you're seeing webbed feet in action: the webbing between its toes works like built-in swim flippers, pushing against the water with each stroke.

The webbing acts like a paddle, giving the animal more power and control in water than separated toes would provide. When a duck pulls its foot forward for the next stroke, it folds its toes together to slip through the water. Then it spreads them wide on the power stroke, using the surface area of the webbing to push backward and propel itself forward.

Not all water animals are web-footed. Herons and cranes, for instance, have long separated toes that help them walk through shallow water and muddy areas without sinking. But for animals that spend lots of time swimming, webbed feet offer an advantage. Some web-footed animals, like penguins, have webbing that helps them swim underwater at high speeds, while others, like pelicans, use their webbed feet both for swimming and for braking when they land on water.