Ingredients:

1 cup water
1/4 cup liquid tempera paint
2 squirts Joy dishwashing detergent
1 Tblsp vinegar
2 cups plaster of paris

Use plastic ice cream trays for molds. Mix all ingredients and pour the liquid into the molds. Let harden for 1 to 2 days depending on the humidity. Pop out the chalk for use. To create multi colored chalk, the colors can be layered.