It's a good thing sloths don't have to go to school. They'd never make it on time. These drowsy tree-dwellers sleep up to 20 hours a day! And even when they are awake, they barely move at all. In fact, they're so incredibly sluggish, algae actually grows on their fur. Sloths live in the tropical forests of Central and South America. With their long arms and shaggy fur, they resemble monkeys, but they are actually related to armadillos and anteaters. They can be 2 to 2.5 feet (0.6 to 0.8 meters) long and, depending on species, weigh from 8 to 17 pounds (3.6 to 7.7 kilograms). There are two main species of sloth, identified by whether they have two or three claws on their front feet. The two species are quite similar in appearance, with roundish heads, sad-looking eyes, tiny ears, and stubby tails. Two-toed sloths are slightly bigger and tend to spend more time hanging upside-down than their three-toed cousins, who will often sit upright in the fork of a tree branch