Nitrogen is one of the primary nutrients critical for the survival of all living organisms. Although nitrogen is very abundant in the atmosphere, it is largely inaccessible in this form to most ...

The water cycle explains the continuous movement of water on Earth. It begins with water evaporating from the surface, rising into the atmosphere, cooling, and condensing to form clouds ...

Several hormones control this cycle - for example, they are involved in controlling the release of an egg each month from an ovary, and changing the thickness of the uterus lining.