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. Three-toed sloths have facial coloring that makes them look like they're always smiling. They also have two extra neck vertebrae that allow them to turn their heads almost all the way around! Some scientists think sloths developed their slow-motion lifestyle so they would be less noticeable to predators such as hawks and cats, which rely heavily on their eyesight when hunting. The algae that grows on sloths' fur also helps them avoid predators by letting them blend in with green leaves. They rarely come down from the trees. About once every week, they descend to go to the bathroom, slowly moving about by digging their front claws into the dirt and dragging their bodies. If they are caught by a predator, sloths turn from sluggish to slugger, biting fiercely, hissing, slashing with their claws, and shrieking.