We just saw that a light stick is a housing for two chemical solutions, which give off light when they are combined. Before you activate the light stick, the two solutions are kept in separate chambers. The phenyl oxalate ester and dye solution fills most of the plastic stick itself. The hydrogen peroxide solution, called the activator, is contained in a small, fragile glass vial in the middle of the stick.

When you bend the plastic stick, the glass vial snaps open, and the two solutions flow together. The chemicals immediately react to one another, and the atoms begin emitting light. The particular dye used in the chemical solution gives the light a distinctive color.

Depending on which compounds are used, the chemical reaction may go on for a few minutes or for many hours. If you heat the solutions, the extra energy will accelerate the reaction, and the stick will glow brighter, but for a shorter amount of time. If you cool the light stick, the reaction will slow down, and the light will dim. If you want to preserve your light stick for the next day, put it in the freezer -- it won't stop the process, but it will drag out the reaction considerably.

Light sticks are just one application of an important natural phenomenon -- luminescence. Generally speaking, luminescence is any emission of light that is not caused by heating. Among other things, luminescence is used in televisions, neon lights  and glow-in-the-dark  stickers. It's also the principle that lights up a firefly and makes some rocks glow after dark.