A desert is a place where rainfall is very uncommon. Because rainfall is unreliable, most plants find it impossible to grow, and only a few special kinds of plants can survive. Usually these plants can store water in their stems (as cacti do), or they have very deep roots.

There are three types of desert: rocky deserts, which are dry and have no sand, stony deserts, where the desert is covered in pebbly stones, and sandy deserts, which are the kind that have sand dunes.

Sandy deserts are not as common as the other kinds of deserts, but they are the ones usually shown in pictures. In this picture you can see sandy desert in the valley, and rocky desert to the sides.

A ridge of curved sand is called a sand dune. Sand dunes form because the wind can blow the sand about making shapes that are not unlike waves. That is why many sandy deserts are called sand seas.

The sand is formed as the rocks of rocky deserts break up into small pieces.

The world's largest sandy desert is in North Africa, and is part of the Sahara Desert.