It depends on the kind of crystals? Any dissolved solid substance will form crystals if it is allowed to precipitate from the liquid that it is dissolved in over a long enough period of time. You can do this yourself: mix a nice wad of salt into some water, heat the water up to dissolve the salt, and then pour the salt water into a mug or a bowl and let it sit an the water slowly evaporate. You will get salt crystals left over. The same thing will happen if you do this with sugar. If you do this with orange juice, you'll get citric acid and ascorbic acid (vitamin C) crystals along with the sugar. You do this with vinegar, you will get acetic acid crystals. And so on: anything that is solid will crystalize.

There are many ways to make crystals. One easy way is to use sugar or copper sulphate. Here is a web site that gives you all you need to know!!!
It is a lot of fun!