No battery. No bulb. No electricity. Just two chemicals meeting for the first time — and releasing their energy as pure light.

When you snap the glow stick, you break a tiny glass vial inside. 

Two chemicals that were kept carefully separated suddenly mix together. 

They react, and instead of releasing heat like most reactions do, they release light. Scientists call this chemiluminescence — chemistry making its own glow.

Now here's where the hot and cold water come in.

Heat gives molecules energy. More energy means they move faster, crash into each other more often, and the reaction runs at full speed. 

The hot water glow stick blazes bright because the chemistry is sprinting.

The cold water does the opposite — it slows the molecules right down, fewer collisions, slower reaction. Dimmer glow. But it lasts much longer.

Did you notice the hot one fades faster? It burned through its chemicals quickly. The cold one is being careful, rationing every reaction.

Did you also notice the snap doesn't create any light itself? The light only starts when the chemicals mix. The snap is just the trigger.

Here's something to try — put a used glow stick in the freezer overnight. You might get a surprise in the morning. 

But fireflies do something almost identical to this inside their own bodies — no snap required. 

To see another chemical reaction releasing energy — but as heat and fire rather than light — try [The Invisible Fire Extinguisher]. And if you want to explore the light itself and how different colours behave, [What Colour is Your Shadow] shows you something equally surprising.

How do they switch it on and off whenever they want? Find out in The Crazy Scientist Lab!