Over millions of years ago, there were no deserts. Now about three tenths of the Earth's surface deserts. The size of the world's deserts is always  (change). Deserts can be hot or cold. The Australian Desert is a hot desert, while the Gobi Desert in Mongolia is an example of a cold desert. Deserts are dry areas  less than 50 cm of rain falls each year. The Sahara Desert in Africa receives less than 1.5 cm a year.  rain doesn't fall very often in deserts, sometimes even for years, there can be heavy rain (sudden). Because they are dry, deserts have wide changes in temperature each day. When the sun (go) up, it is very hot. At night, the heat  (receive) by the ground during the day  (quick) disappears (消失), and then it becomes very cold. Becausethe change, living in the desert is very difficult. Man can make deserts, but they can also stop them from getting larger and larger. Planting trees can prevent the sand from(spread).