Deserts are found where there is little rainfall or where rain for a whole year falls in only a few weeks’ time. Ten inches of rain may be enough for many plants to survive if the rain is spread throughout the year. If it falls within one or two months and the rest of the year is dry, those plants may die and a desert may form. Sand begins as tiny pieces of rock that get smaller and smaller as wind and weather wear them down. Sand dunes（沙丘）are formed as winds move the sand across the desert. Bit by bit, the dunes grow over the years, always moving with the winds and changing the shap