I make sidewalk paint a little differently than the typical mix of cornstarch + water + food coloring. I mix 1/2 cup of cornstarch in 1 cup of cold water. Then I put three cups of water in a pan, add a teaspoon of salt, and heat to boiling. I turn the heat down to low and slowly stir in the cornstarch and water while constantly mixing. It quickly becomes thick and gelatinous. I pour it out into individual containers and let it cool a bit before mixing in the food coloring.
Isn't it pretty? The girls had a great time, although for a while they were more interested in painting the ants than being artistic!