A hippopotamus is a massive, thick-skinned mammal that spends most of its life in African rivers and lakes. Despite looking slow and clumsy, hippos are surprisingly fast and dangerous: they can run faster than humans on land and are considered one of Africa's most aggressive animals. Their name comes from ancient Greek words meaning “river horse,” though they're actually more closely related to whales than to horses.

Hippos can weigh up to 4,000 pounds (about as much as a small car) and have enormous jaws that can open nearly 180 degrees. They use their giant teeth, which can grow over a foot long, to fight other hippos and defend their territory. During the day, hippos stay mostly submerged in water to keep their skin from drying out in the hot sun. At night, they lumber onto land to graze on grass, eating up to 80 pounds in a single evening.

A group of hippos is called a pod or a bloat. Baby hippos, called calves, are born underwater and must swim to the surface to take their first breath.