Two parts Plaster of Paris (from a craft store, i.e. Michael’s)
One part water (or just a little less)
Coloring - Powdered or liquid tempera paint or food coloring (note: don’t use too much food coloring or it could stain your sidewalk ~ just a few drops)
Measure Plaster of Paris into a bowl (margarine tubs work well), then gradually add warm water, stirring to combine. Add coloring, then allow mixture to thicken. Add more water, if required, to consistency of thick frosting. Pour into molds (Dixie cups; plastic ice cube trays; empty pill boxes or film containers work well). Pour mixture into molds and let dry.
This can take anywhere from several hours to a day (maybe longer), depending on size of the mold. (Remember: The bigger the mold, the longer it takes to dry.) Once dry, remove chalk. If chalk is still moist, let it air dry for another 24 hours.