A beaver's front teeth never stop growing. They have to chew wood all the time just to keep them the right length. The teeth are orange because they contain iron, which makes them really hard - hard enough to cut down a tree as thick as your wrist in a single night.
Beavers build dams across streams using sticks, mud and stones. The dam blocks the water, so the stream backs up and forms a pond behind it. The beavers then build their home - called a lodge - in the middle of the pond. The lodge is a big dome of sticks with the door underwater, so foxes and wolves can't get in.
Inside the lodge, a beaver family has a dry room above the water level, lined with soft chewed wood. Mum, dad and their children all live together. Beaver children stay with their parents for about two years before going off to start their own families.
Beavers are amazing for rivers. Their dams slow the water down, create new ponds, and bring back fish, frogs, ducks and dragonflies. Like the bison, beavers had nearly disappeared from Germany. Today they are back in many rivers - and rangers are watching new ponds appear all over the country as beaver families set up home.