Exfoliation is a process of weathering in which rock flakes off in small pieces. It is caused by large differences in day and night temperatures, as occur in hot deserts. During the day the rock heats up and expands; during the night it cools and contracts. Eventually the rock is weakened by the repeated expansion and contraction, and pieces flake off. The presence of dew or other water hastens the process. Because the rock flakes off like the skin of an onion, it is also known as onion weathering.