Many natural substances that are necessary for life exist in cycles in which they are used and re-used.

• Rocks can be changed from one type to another. This is called the rock cycle.
• The water cycle describes how water circulates from the ground, to the air and then back to the ground again.
• The nitrogen cycle describes the process in which nitrogen gas is converted into usable nitrogen compounds and then converted back to nitrogen gas to be re-used.
• Carbon is cycled from the atmosphere and oceans, through organisms and then back into the atmosphere and oceans.