You will need:
1. Snap a chalk piece in half
2. Soak the chalk piece in water until no more air bubbles rise from the chalk - as a precaution, move the chalk around to shake loose any surface air bubbles clinging to the chalk sides.
2. Remove the chalk from the water and place the chalk into the plastic bag.
3. Place the plastic bag in the freezer and wait 6 hours.
4. Pour hot water into a bowl and place the now frozen chalk into it. Ideally a hot dry oven dry would be better as the thermal change would be greater. If you do use an oven, you will need to re-soak the chalk again.
5. Repeat steps 2 to 4 until the chalk breaks