Beavers build them from sticks. Landslides create them from trees, mud, and debris. Humans make them from earth and concrete. These structures are dams. Dams hamper the flow of water in a river or stream.

Landslides don’t mean to create dams. They do so by dumping a lot of earth and other stuff in a river. Scientists think beavers build dams for protection. Beaver dams capture water in front of the lodges in which beavers live. Beavers can hide from their enemies in this deeper water. A dam also protects the beaver lodge by slowing the river’s speed.

Imagine you are snorkeling in the warm ocean water around a tropical coral reef. You put on your mask and swim fins. You float around looking at the reef below you. The coral reef looks like strangely shaped rocks sticking up from the ocean floor. Crooked coral that looks like tree limbs twists up from cracks and caves in the reef. A big piece of coral that looks like a brain lies on the bottom. Orange and pink coral shaped like fans wave in the currents. Fish of all different shades and varieties of color swim near the reef.

Coral reefs play an important role in ocean life. Many kinds of plants, fish, and other animals live on and around a coral reef. Even the reef itself is made mostly of tiny coral animals, both living and dead.

You have never watched a dodo bird scurry along the ground. You have never been frightened by a saber-toothed cat crouched in a tree. You have never seen the huge tusks of a mastodon. These animals once lived on Earth, but they all died out. They went extinct.

You can see still see blue whales, giant pandas, and tigers. But these animals are in danger of becoming extinct. There aren’t many of them left. Blue whales, giant pandas, and tigers are some of the endangered animals on Earth. There are thousands of others.