Fireflies light up using a process called bioluminescence, which is a chemical reaction that occurs inside their bodies. It’s the same basic principle that makes some mushrooms glow in the dark and deep-sea creatures light up. The firefly’s abdomen contains a chemical called luciferin and an enzyme called luciferase. When these two chemicals react with oxygen, they produce a flash of light without generating heat. The firefly can control the timing of these flashes by regulating the flow of oxyg