An otter's body is built for swimming. Its tail works like a rudder and a paddle. Its webbed feet push it through the water. Its nose and ears close up tight so no water gets in. It even has special see-through eyelids that work like swimming goggles, so it can see clearly underwater.
Otters hunt mostly fish, but also frogs, water birds, crabs and the odd water vole. They can stay underwater for nearly a minute, twisting and turning to catch a fish in their mouth. Once they have caught one, they often climb out onto a log or stone to eat - they like a dry table.
Otters are very playful. Scientists who watch them say playfulness is a sign of a very smart animal - they spend lots of time on things that aren't strictly necessary, like sliding down a muddy bank again and again. Otter cubs especially love to wrestle and chase each other through the water.
Denmark lost most of its otters in the 1900s because the rivers were too polluted - the fish died, so the otters had nothing to eat. Since then, Denmark has cleaned up its rivers, and otters have come back. They are still rare, but spotting one in the wild is one of the great wildlife treats in Denmark.