How do fire flies light up?

Fireflies glow because they have different chemicals in their bodies. When combined, there's a chemical reaction and they glow.

To be more specific, cells in the firefly's abdomen produce luciferin and that creates luciferase, an enzyme. The luciferin combines with oxygen and forms oxyluciferin, which is inactive, but luciferase speeds up the reactions. Luciferin combines with ATP (adenosine triphosphate) and luciferyl adenylate and PP(pyrophosphate) are produced.

Oxygen and luciferyl adenylate mix to produce oxyluciferin and AMP (adenosine monophosphate). Finally, it starts to glow.

There are also uric acid crystals produced by the abdominal cells that help the light reflect away.