Finally today we did the hot crayon rocks that the boys have been waiting ages for. We collected them a few weeks back when we went fishing and they've been sitting in a bowl on the table ever since.
It's easy and very effective.
1. Heat the rocks in the oven (around 200 degrees).
2. Handle them carefully with oven gloves, place them on to cardboard or thick newspaper.
3. Draw on them with crayons. Fun!
WARNING make sure your rocks are super dry before doing this... wet rocks might explode in the oven!!