Explain that inside the plastic of the light stick is a glass rod. One chemical is inside the glass rod and another is outside. When you snap the glass rod by bending the light stick, the chemicals mix together, and the chemical reaction produces a new molecule that glows.
Turn the lights out, then ask students to snap their sticks all together. Ask students to look closely as the chemicals mix - they should be able to see a swirl of colour as the chemicals react and new coloured glowing molecules are made.
First photo shows the chemicals mixing and making light.
(Older students: the tube contains hydrogen peroxide, which mixes with a chemical outside the tube. Two chemical reactions result in the dye getting excited as it gains energy. It releases this energy again as light. Additional dyes make the glow sticks different colours. See Wikipedia entry for more details).

Ask students to find out how heat and cold affect the rate of the chemical reaction in their light sticks, by dipping the light sticks in the warm and iced water tubs.
They should have found that in the warm water the light stick glows more intensely. This is because the heat energy causes the molecules in the light stick to move around faster, hence collide more often and chemically react to make more glowing molecules.
The light stick dipped in cold water should become dim. Heat energy leaves the lights stick and moves into the cold water, hence the molecules in the light stick have less energy and move around less. The cooler molecules collide less frequently, so undergo fewer chemical reactions to make glowing molecules.
(Sometimes students see something different, possibly due to the intensity of the glow stick affecting the sensitivity of our eyes in the darkness. These are real results as they are what the students found, but during discussion focus on what the majority of the students found.)

Students can make bright and dark stripes along their glowstick by dipping in one tub, then dipping it not as far into the second tub.

Discuss how the chemicals are used up when it glows brighter, and how they can preserve their glow sticks by putting them in the freezer overnight to slow the chemical reaction down.