Today M and I found a big rock in the backyard and heated it in the oven. When it was nice and toasty I took it out, and we began (carefully) coloring on it with crayons. The effect was really unique. The crayon tips melted instantly, and suddenly it was like painting with hot wax.
M and I just kept saying "This is so interesting!" to one another as we tried different colors. After we painted the rock, we tried an old plant pot. That worked nicely too; you can see that when the colors mixed together it created a nice marbling effect. Unfortunately, the marbling effect became less pronounced as some of the wax got absorbed into the dark pot.
I was only willing to do this activity with M because she is an especially cautious little three year old and I could trust her not to touch the rock. It was hot! Also, next time I will make sure we all wear painting clothing. The sleeves on M's shirt don't look so good, and I don't think it's going to come out...oops.