More than millions of years ago, there were deserts. Now about one fifth of the Earth's surface ____ (be) deserts. The size of the world's deserts keeps ____ (change). Deserts can be hot or cold. The Australian Desert is a hot desert, while the Gobi Desert is a cold desert. A desert is a very dry land and there is hardly any rain. The Sahara Desert in Africa receives less than 1.5 cm a year. ____ it doesn't rain very often in deserts, sometimes even for years, there can be heavy rain suddenly. Because they are dry, deserts have wide changes in temperature each day. When the Sun goes up, it is very hot, even hot enough ____ (fry) an egg on the ground. At night, the heat that ____ (receive) by the ground during the day disappears ____ (quick), and then it gets ____ cold that water would turn into ice. Because ____ the change, living in the desert is very difficult. Man can make deserts, but man can also stop ____ (they) from getting larger and larger. Planting trees can prevent the sand from ____ (spread).