A sloth moves about 40 metres a day - that's roughly the length of one school corridor. The reason is their food: they eat mostly leaves, and leaves take a very long time to digest. A sloth's stomach takes around a month to finish one meal. So they don't need much energy, and they save it by hardly moving.
Sloths spend almost their whole lives upside down, hanging from branches by their long curved claws. They sleep upside down. They eat upside down. They even give birth upside down - and the baby clings to mum's belly straight away.
Sloths move so slowly that tiny green algae grow in their fur. The algae give them a slightly green tint, which helps them hide in the leaves. It is the only animal-and-plant team-up like it on Earth - the sloth gets camouflage, and the algae get a moving home.
The three-toed sloth's face is curled into a permanent gentle smile - just the way its mouth is shaped. They look as though they are always in a good mood, even when they are asleep. Costa Ricans have a deep fondness for sloths, and they have become a kind of national mascot.