In a mixing bowl, combine the water and cornstarch and stir until the cornstarch dissolves and it is smooth. It will be kind of runny.
Divide the mixture evenly into the small plastic cups. Use the food coloring to make a different color paint in each cup.
Use paintbrushes to paint pictures on the sidewalk or driveway outside. The mixture is thinner and runnier than regular paint, but just paint your pictures like you would with any other paint. The paint dries quickly, and it will resemble sidewalk chalk. The colors are very bright once they dry! It washes off very easily with rain or the hose.