How Do Fireflies Glow?

Glow worms (which later become fireflies) glow because of a chemical reaction between complex molecules and oxygen in their bodies. This chemical reaction is very efficient--almost 100% of the energy produced by this reaction is given off as light, which is much more efficient than a lightbulb! Why do they glow? Some scientists think it's a warning sign to predators not to eat them! Imagine a restaurant with a big neon sign that said "BAD FOOD SERVED HERE." It's the same idea.