How Do Corals Build Reefs? | California Academy of Sciences?

Corals build reefs by working together in teams, just like how you and your friends might build a tower out of blocks.

Corals are tiny animals that live in the ocean. They're so small, you can’t see them with just your eyes, you need a magnifying glass or even a microscope! But when they team up with tiny plants called algae, something amazing happens: they start to grow bigger and stronger.

Corals use special tools, like little cement machines, to make a hard shell around themselves. This hard shell is called calcium carbonate, which is the same stuff that makes up seashells! Over time, as more corals join in, they create big structures that look like underwater castles.

These coral structures grow slowly, about an inch every year, but over hundreds or even thousands of years, they become huge reefs. It's kind of like when you and your friends build a sandcastle at the beach: each person adds a little bit, and soon it becomes something big and beautiful.