The deserts of the world are not all covered with sand. Many of them have surfaces of rock or clay or small stones. They are not flat, either. They often have high hills and deep valleys. There is some plants' life in many parts of the desert. There is little rain in the desert, but it does fall often enough for most plants. The deserts of the world are not uninhabited (not lived by people). People also live outside oases(绿洲), but these people are not farmers. They have camels, goats, donkeys, sheep, etc. These animals can live on the desert plants and do not need much water. The people of the desert have to move constantly from place to place, or they must always look for grass or desert plants for their animals. They usually live in tents. When there is no more food for their animals, they fold up their tents, put them on their camels and donkeys, and move to another place. In good years, when there is enough food for their animals, they trade their skins and their goats and camel hairs with the people of oases for wheat and fruit. But in bad years, when there is not enough food for their animals, the people of the desert would attack the oases people. But they are also hospitable, no man in the desert would ever refuse to give a stranger food and water. In the desert______.