The sea otter is a marine mammal that spends most of its life floating on the surface of the ocean. The soft, dense fur which can hold air bubbles within it, and the sea otter’s large lungs mean they can easily float on top of the water. They are the only mammals, apart from primates, that use tools, and they use rocks to break open sea urchins, abalone, and other shellfish to eat, as they float face up in the water. Each sea otter has its own favorite rock to use, which is kept safely in a pouch on the otter’s left side. In the early 20th century, sea otters came close to extinction due to overfishing and oil spills from grounded tankers. They have now been granted protection and numbers are on the increase, but still retain status as an endangered species.