i found that out on my trip to botswana. you know that?nti river in botswana's okavango delta is hippo heaven. [hippos grunt] my guide, andrew correll, knew just where to find them. that's a big one. boy, i never seen so many hippos. do they stay in the water all day long? >> yeah, this is their natural habitat. >> hippos beat the heat by staying in the water about 16 hours a day, and they need lots of sun block. but who would be crazy enough to rub it on? fortunately, no one needs to. the hippos' own bodies produce a red, oily sweat that keeps their sensitive skin from getting burned. they are so well-adapted to the water, they can even go to sleep while completely submerged. >> what's incredible, though, is that when they're in deep water and they sleep, they have the ability to rise up, breathe, and sink back down again underwater without waking up. [african choir harmonizes] >> hippos dine on grass, not people... but even so, they should never be approached when out of the water, especially after dark. >> because of the fact that they