I saw a great tutorial for making Homemade Sidewalk Chalk on Martha a few weeks ago and thought I'd give it a try. It's super easy and we really enjoyed it. We liked the idea of making shaped chalk so we used silicone baking trays and even some plastic chocolate molds. All you have to do is follow the directions on the package for Plaster of Paris and add a little dry tempera paint to the mix.
This chalk does NOT work on chalkboards because it's a little too hard, but it works great on sidewalks.