One 4.5 tonne hippo opening its jaws to 150 degrees and showing a row of blunt-ended teeth is enough to frighten anyone - but sometimes a herd of more than 100 gather together.
They 'yawn' not because they are tried, but to warn others or attract a female.
Hippopotami are commonly found in African rivers and marshes. They spend most of the day submerged in water, with just their eyes, ears and nostrils above the surface.
Hippos are seldom seen on land during the day because their skin quickly loses moisture in the sun. To counteract this, glands secrete a reddish film of sweat which covers the body and helps cut out ultraviolet radiation.
If you happen to see a pink hippo, it is not sunburnt or sweating blood - just using protective suntan lotion.
In the evening, hippos come out on land to feed. With teeth which act as cutting and grinding machines, hippos can eat tough spiny grasses that are unsuitable for goats or zebras. This is important as they are not competing for the same food source and so can live peacefully with other grass-eating animals.