Yes, it's really true - and quite simple, actually.
* Small yogurt cups and cut the tops off to make tubs or disposable cups or toilet paper rolls stood up on a piece of paper.
* Plaster of Paris
* food coloring
Put your plaster of paris into a bucket. Add a little bit more than the same volume in water. Stir like crazy - it should have the consistency of soft-serve ice-cream. Add your drops of food coloring and stir some more. Pour the mixture into your containers. And leave them to dry overnight. If you are making rainbow chalks then pop the chalks into a cup, one color on top of another. If you want your chalks to be less marbled then let them set between layers. Patience is not an easy taskmaster and our always have a more very marbled look... as we just place the colors on top of one another. Once your chalks have set overnight, then just peal the cups off and if you made them in plastic tubs then just cut them out using kitchen scissors.