On an elementary school playground, hundreds of children run and jump, smiling and laughing as they play. Watching sea otters  playing in their own environment is like watching children at play. Sea otters, which are presently on the endangered species list, are experiencing an increase in population. Groups of sea otters live on the shores of the North Pacific Ocean. They are seldom found far from land but almost spend their entire lives at sea. The otter has been known to dive as deep as 180 feet and stay underwater as long as minutes. Each day the sea otter eats as much as one-fifth of its body weight in fish or shellfish. Not only are sea otters playful, they are also amazingly resourceful. Their favorite food, abalone  , is protected by a thick shell that sea otters are unable to break with their teeth. Sea otters use stones from the ocean floor and break the abalone open on their bellies  as they move slowly across the surface of the water on their backs, a behavior that makes them one of only a few animals on Earth, including human beings, to use tools! Their intelligence is also displayed while sleeping. In order not to move into dangerous waters, sea otters actually cover themselves with plants on the sea, which keep them in place before falling asleep. Because sea otters have a special means of keeping insulated , they can keep their body heat in their cold environment. They have unbelievably fine fur  that traps tiny bubbles, and that keeps them comfortable when warmed by body heat. However, their fur loses this insulating quality if it becomes dirty. Sea otters have faced dangers from hunters in the past and from oil spills more recently. Fortunately, due to a 1911 agreement and to rescue and habitat protection efforts, the sea otter population has nearly recovered. What can be inferred about sea otters' fur?
It consists of tiny air bubbles. It has a layer of insulating water bubbles. It keeps sea otters warm only when it's clean. It becomes dirty easily in the cold environment.