Alright this is an old one. I learned this in grade school, they probably don't do it anymore because it involves dangerous chemicals like water. But it's fun for the kiddies.
Step 1: You will need
Laundry Bluing, getting hard to find, it's a suspension of Prussian Blue in water.
Clear Household Ammonia
Toilet Paper Roll Core
Step 2: Procedure
Mix equal quantities of water, ammonia, salt, and laundry bluing together. Pour solution into a bowl and stand a tp roll in the bowl. Over the next 18-24 hours the crystals will form.
Step 3: Optional steps
Optionally the cardboard, and subsequently the crystals can be colored with drops of food coloring.
Other porous materials can also be used such as terra cotta and coal (the traditional substrate).