Otters have webbed paws and dense, waterproof fur. There are around 70,000 hairs in every square centimetre of an otter's coat - more than on any other animal in the world. All those hairs trap a thin layer of warm air against the skin, keeping the otter dry and toasty even in icy mountain rivers.
They eat mostly fish, which they hunt by ducking under and chasing them with rapid twists. They can hold their breath for around half a minute and often surface with a fish wriggling sideways in their teeth. After a meal, they will sometimes lie on the bank and roll in the grass to dry off.
Otters in Slovakia almost disappeared 50 years ago because the rivers got dirty. As the country cleaned up its rivers and streams, otters came back. Today they live across most of the country, including in the rivers that pass through cities. Spotting one is rare, because they are mostly active at dusk and dawn.
Otter families like to play. Cubs slide down muddy banks, chase each other underwater, and 'wrestle' on the riverbank. Even adult otters seem to enjoy a slide for no reason. Some scientists think this play helps them practise the quick movements they need for hunting and escaping danger.