They gnaw through bark to eat the sugary layer underneath.

They've been gnawing through forests for 20 million years.

Felling trees with your teeth takes great strength, skill, and patience.

One technique is to slice halfway through and let the wind do the rest.

The beavers' incisors are strengthened with iron, which makes them orange.

They grow continuously and even self-sharpen.

The pond makes it easier to move around the heavy logs they need to build their dams.

Out of the water, it's a struggle.

Stones help weigh down the base.

The whole family works together, carefully interlocking the timber.

They dredge mud from the pond bottom to seal the dam.

Each pond traps several inches of sediment every year so there's plenty of it.

The young act as apprentice builders, learning the tricks of the trade.

The final results are impressive.

In the Rocky Mountains beaver dams slowly filter billions of tons of water.

The ponds build up soil and nutrients and help prevent floods and droughts.

(Leave it to Beavers, Wednesday, May, 8/7c on PBS)