coral reef

• An animal called 'polyp' builds itself a skeleton from the lime it collects from the sea. 
• A polyp looks like a plant and lives in warm salt water. 
• The skeleton it builds is as hard as rock.
• It protects the polyp during its life time.
• When the polyp dies, the skeleton remains. 
• A coral reef is formed when so many polyps join together over thousands of years and left the skeletons behind.

Source: google images