Then pour 3/4 cup of warm water into a disposable plasic container. Sprinkle in about 1 1/2 cups of plaster of paris, a little at a time, until the powder is all dissolved. Stir with a spoon.
Then mix in 2 to 3 Tablespoons of powdered tempura paint. (Rinse your things later under an outside faucet since plaster of paris can clog your drains. Not fun.) Now pour the plaster of paris liquid into the yogurt container carefully and slowly. Tap the sides after so that air bubbles will disappear.
Now, let the molded chalk harden for a couple of days. After a that, you should be able to slide the homemade chalk out of the yogurt container. Try it with shaped muffin tins! Your kids will love them!