First and foremost. Do not attempt this unless you fully understand the risks and dangers behind it. It is not as easy as it looks. Glass can break and shatter if hit wrong of getting mad and throwing it cutting your hands or wrists. The force you hit the glass with, applied to the wrong part of your hand, can break it. Make sure you have band aids nearby and gauze. I personally would recommend this only to adults, being the higher strength they can produce, and nothing against the children, but they assume responsibility a lot quicker than younger folks.
Items needed for this project....
1. A Glass Bottle
3. The cap to the glass bottle
4. Any safety precautions you wish.
Step 2: Filling the Bottle
Fill the bottle with water until right about the neck area. Any higher than that and it becomes extremely difficult to break. Tighten the cap on the bottle. You don't have to over tighten it or wrench it out or anything, but tight enough to stay.