A hippo's skin looks tough, but it is actually quite sensitive to the sun. To protect themselves, hippos produce a reddish, oily liquid from their skin that acts as a natural sunscreen and keeps their skin moist. Scientists once thought it was sweat, but it is a unique substance found nowhere else in the animal kingdom.
Hippos are surprisingly good swimmers. Despite weighing up to 3.5 tonnes, they can walk along river and lake beds, bounce off the bottom to rise to the surface, and even 'run' underwater in slow motion. Baby hippos can swim before they can walk well, and they are sometimes born underwater.
In the Nile and the Sudd, hippos play an important role. Their dung (poo) falls into the river and feeds algae and fish - turning them into underwater fertiliser factories. Nile tilapia, the fish that South Sudanese families love to eat, partly depend on this nutrient cycle.
Hippos live in groups called pods or bloats. The group spends the day clustered together in the water, ears twitching, occasionally yawning to show their enormous tusks. A hippo's yawn is not tiredness - it is a way of saying 'I am here and I am very large.'