Hippos spend most of each day in shallow water, emerging at night to travel to "hippo lawns," grassy areas where they graze. Grazing only at night allows them to keep their skins moist and out of the African sun. When they're not grazing on grass—which at night takes them into the African lowlands several miles away from the water and for periods of five or six hours at a stretch—hippos prefer to spend their time fully or partially submerged in freshwater lakes and rivers, and occasionally even in saltwater estuaries. Even at night, some hippos remain in the water, in essence taking turns at the hippo lawns.