Hippos spend most of the day in the water. Their thin pink-grey skin would burn in the African sun, so they sit in rivers and lakes to stay cool. They can hold their breath under water for around five minutes. Babies can even breastfeed underwater - they pop up for a quick gulp of air, then dive back down.
At night, when the sun has set and the air is cool, hippos leave the water. They walk for kilometres along well-worn paths called 'hippo trails' to munch on grass. A single hippo eats about 40 kilograms of grass each night. Then before sunrise they walk back to the river.
Hippos look slow and lazy but they can be surprisingly fast. On land, an angry hippo can run at over 30 km/h - faster than the world's fastest human. Their teeth, especially the canines, can grow over half a metre long. They never stop growing throughout the hippo's life.
A group of hippos is called a 'pod' (like dolphins) or a 'bloat' (because they look so round). The older male is in charge of the pod, but the females and babies do most of the chatting. Hippos talk to each other with a deep booming noise that travels through the water for kilometres.