Deserts:

Today, about 20 percent of Earth's land surface is desert. Deserts occupy land on every continent except Europe.  Although each desert is notable for its unique landforms and terrain, all deserts share a common condition, aridity.  or lack of water. Aridity is measured primarily in annual rainfall.  Most deserts recive fewer than 10 inches f rainfall yearly; some go for years without ever receiving rain. 

Many deserts plants and animals have hightly evolved behaviors that enable teh  to conserve water in these exceedingly dry climates.  THe saguaro cactus and the kangaroo rat in North American desers; plants and camels int eh Sahara; marsupials in the Australian outback; gazelles and antelopes in the Gobi; and penguins in patagonia have all adapted to their environments so that they can survive and reproduce.  Some people, including the San in the Sahara and the Aboriginies in Australia have also adapted to the desert.