Most people are familiar with three states of matter: solid, liquid and gas. Actually, if you dig into the world of physics, there are several more, but for now we will only add plasma to the list, and we will look at the first three states before talking about plasma.
Things like rocks, wood and ice are solid. Solids stay the same size and shape, no matter what container we put them in.
Things such as alcohol, oil, and water are liquids. They stay the same size, but they change their shape to fit their container.
This is another of those fun bits of science that many of us think we understand until we really start to look at it, or even better, try to explain it to someone else. Then we reach a point where it becomes obvious to ourselves, and to our audience, that we don't understand it as well as we thought we did.
One way that crystals form is from chemicals dissolved in water. If the water gets cooler, dissolves other chemicals, or evaporates, some of the dissolved chemicals can be deposited as crystals. Often, growing crystals can be a fairly long, involved process, but with this activity, we will grow some nice crystals quickly and easily.
This experiment got its start while I was reading Craig F. Bohren's "What Light Through Yonder Window Breaks." It is a book on atmospheric physics, and is written so that you don't have to be a physics professor (or even a physics student) to understand and enjoy it. He writes about the dew that forms on your house windows in the winter, which made me think of other questions about dew drops.
This experiment is another old classic which is still a lot of fun. Now that I think of it, it seems that most of the science tricks I did as a kid have become OLD classics, but this was already an old classic even way back then.