Copper sulfate makes the most beautiful bright blue crystals. This project should absolutely NOT be done with small children. Copper sulfate is extremely toxic. It is commonly used to dissolve tree roots in plumbing pipes. With that warning, you can make some amazing crystals.
1.) Begin by purchasing plumbing root killer. This can be found on the plumbing aisle at Home Depot. Just make sure you look at the ingredient list on the back. It should primarily be composed of copper sulfate. You will begin by making a saturated solution. You do this by filling a small glass half way with really warm water. Stir in copper sulfate until no more of the copper sulfate will dissolve.
2.) Pour the solution into a shallow dish. Place the dish in a sunny spot. I prefer a high windowsill because it is toxic and I know no one will disturb it. Once the liquid has dissolved you will be left with tiny blue crystals.
3.) If you want to create much larger crystals. These tiny crystals will serve as your seed crystals. Take the largest of your small crystals and tie a string around it. Once again make your saturated solution. When the solution has completely cooled and is at room temperature submerse your string in your saturated solution. Tie the end of the string to a pencil so the seed crystal is fully submersed in the saturated solution dangling just above the bottom of the cup. After a few days you will notice crystals joining with your seed crystal. Eventually, you will be left with one large blue crystal.