The recipe is one part corn starch, to one part water, with liquid food coloring added, to the desired color. When you paint on the sidewalk with it, it starts out kind of watery, with very little color. But then, as it dries, it takes on the appearance of normal sidewalk chalk. My two youngest loved it.
In case your wondering too, the paint washes away, just as easily as normal sidewalk chalk. And, after it dries, it brushes right off of clothing, and little hands, and feet. I'm not sure though, what the permanent effects might be, for the passing ants, and spiders, receiving a good dousing of color, and starch.
It's great to be a homeschooler.