While the hippopotamus may look like a very gentle animal, it is actually quite dangerous. In fact, the hippo has killed more people than any other wild animal in the world! Their teeth are very sharp and can grow up to 125 centimeters long. Still most people need not worry since hippos are vegetarians and only attack when they are threatened.The hippopotamus spends most of the day sleeping in the mud. During the night it wades in the water looking for food. The hippo is very comfortable in lakes and rivers and can actually spend up to 15 minutes under water. Recent research suggests the hippo is more closely related to the whale than to land animals such as the elephant. Baby hippos are even born in the water and so learn to swim immediately. It's no wonder the word hippopotamus actually means "river horse."