Deserts cover about 30% of the land. Desert climates are characterized by the fact that water is unavailable for living things. Some deserts are hot, with very little rainfall. Some deserts are not hot. These "cold deserts" receive some of their rain at a time when it is too cold for plants to use it. Organisms living in deserts have adapted to the harsh conditions. Plants such as cacti retain moisture in their stem tissues and have eliminated leaves to reduce water loss. Animals living in the desert, such as jack rabbits and kangaroo rats, have adaptations for water conservation including behavior, such as feeding at night, and physiology, such as very efficient kidneys.