PLANET EARTH Deserts
Like the elephants, the lions must travel great distances to find enough to live on.But lions can't go everywhere - they won't attempt to cross this field of sand dunes and the oryx（羚羊） know it. The lions must wait for the oryx to leave the safety of the dunes, which eventually they must to find food and water. And then the lions will ambush（埋伏） them.
The elephants have found some of their favorite food. Grasses are the staple diet（主食） of all elephants, but this herd concentrates on digging up the roots, which have more nutrition and moisture than the stems. It's the sort of behavior that can make all the difference in a place of serious shortages. Yet all this can change in an instant.
The fortunes of many deserts are ruled by distant rains. This water fell as rain in mountains more than a hundred miles away. It's known as a flash flood and called that because the water may run for just a single day. It's an event that only happens once or twice a year at the most. The sandy riverbed acts like a giant strip of blotting paper sucking up the water as soon as it appears.
