Easier - Coral is a substance that is formed by the skeletons of sea animals. Live, healthy coral is often brightly colored. Large coral groups form rounded or branching masses. A strip or ridge of solidified coral at or near an ocean surface is called a coral reef.

Harder - Coral is a limestone formation formed in the sea by millions of tiny animals called polyps. Most coral polyps live together in colonies. They attach themselves to each other with a flat sheet of tissue that connects to the middle of another polyp body; half of the coral polyp extends above and the remaining half is below the connective sheet. Coral polyps remove calcium out of the sea water to build their limestone skeletons. They then deposit calcium carbonate (limestone) around their lower body halves. When the animals die, they leave limestone "skeletons" that become the foundations of barriers and ridges called coral reefs. As new polyps grow, their mass makes the limestone formation larger and larger.

Diverse coral formations may resemble branching trees, large domes, small irregular crusts, or tiny organ pipes. The living coral-forming animals color the formations in beautiful hues of tan, orange, yellow, purple, and green. Coral animals cannot live in water cooler than 65 °F (18 °C), therefore coral reefs are found mostly in warm, shallow, and tropical seas.