You're walking on a beach at night when the waves start glowing electric blue. You splash your hand in the water and it sparkles like someone dumped a million tiny glow sticks into the ocean. What just happened?

The glow comes from millions of tiny living creatures called dinoflagellates — single-celled plankton so small you'd need a microscope to see one. They float in the water by the billions, invisible during the day, but when something disturbs them at night, they light up.

Inside each dinoflagellate is a chemical called luciferin (from the Latin word for light-bringer). When the creature gets jostled — by a wave, a fish, your hand — luciferin mixes with oxygen and an enzyme, and boom: cold light. No heat, just glow. It's like a glow stick that resets itself.

But why do they do it? Scientists think it's a burglar alarm. When a small fish tries to eat the dinoflagellates, the sudden flash of blue light says "Hey! Something's eating over here!" That attracts bigger predators who might eat the fish instead. The dinoflagellates turn their enemy into bait.

The effect only happens when billions of dinoflagellates bloom together in warm coastal waters. You can't just find it anywhere. California beaches, Puerto Rico's bioluminescent bays, parts of Thailand and Australia — these are the famous spots where the conditions line up just right.

And it's not just dinoflagellates. Jellyfish, squid, deep-sea fish, even some bacteria and fungi glow using similar chemistry. About 80 percent of deep-sea creatures make their own light down where the sun never reaches. The ocean is full of living lanterns.

You can sometimes see bioluminescence in breaking waves during the day, too — the water looks reddish-brown from all the dinoflagellates packed together. That's called a red tide. At night, though, that same rusty water transforms into liquid starlight.

So the next time you're at the beach on a dark night and the water starts glowing around your feet, you'll know: you're standing in a galaxy of tiny creatures, each one flashing its microscopic light to say "Don't eat me!" And together, they turn the whole ocean into magic.