*Glow sticks work because of a chemical reaction.
Glow sticks contain two substances - one housed in the plastic casing and another inside a thin glass tube within the plastic. When the stick is bent, the glass breaks, allowing the two substances to react with one another. More information can be found here.
*The speed at which a chemical reaction occurs is dependent upon the temperature at which the reaction is occurring.
This is not the only factor that influences rate of reaction, just the only one we'll be talking about here.
For today's experiment:
You'll need two glow sticks* (two of the same color is best - makes it easier to compare), 2 glasses, hot water and cold water.
The water can come from the tap. I usually stick the glass of cold water in the freezer for a minute or two while I'm getting everything else ready, just to make it extra cold. I get as hot of water as I can from the tap. I suppose you could boil some water, but this works just fine and reduces the prep work.
Put your two glasses next to each other. Activate both glow sticks at the same time and place one in each glass.
Left side = hot water, Right side = cold water
After a minute or two, you'll notice that the stick in the hot water is glowing much brighter** than the one in the cold water. In fact, the one in the cold water will barely appear to be glowing.
The reaction that causes the glowing is occurring at a much more rapid pace in the hot water, thus the stronger glow.
Your students may have heard that they can save their glow sticks by placing them in the freezer. Doing so will slow down the rate of reaction, so the glow stick will last longer; but it doesn't stop the reaction, once the glow stick has been activated, it won't last forever, even in the freezer.
*Keep your eyes on the dollar section of stores such as Target - you can occasionally find 12 sticks for a dollar there - makes it affordable enough to consider doing in the classroom.
**There really is a drastic difference in the 'amount' of glow being emitted from the two tubes, I promise. It doesn't photograph very well, at least not with my limited photography skills. So try it for yourself.