Corals are tiny animals, called polyps, that are related to and look like sea anemones.
Each coral secretes a stony cup of limestone around itself as a skeleton. The polyps divide as they grow and form coral colonies. As the coral colonies build up on top of each other, they gradually form a coral reef. Individual coral colonies may be up to 1000 years old. Coral reefs may be many thousands of years old, forming slowly over time, responding to changes in sea level and other environmental conditions.