Deserts have less than 25 cm (10 in) of rain a year. They cover about a fifth of the Earth's land surface. Deserts are sandy, rocky or covered in stones and gravel. The Sahara Desert is almost as big as the whole USA. The temperature there may reach 57°C (134°F) during the day and drop well below freezing at night. Many plants and animals live in deserts. They all face the same problem finding enough water to stay alive.