Combine 1 cup cornstarch and 1 cup water in a non-slip mixing bowl. Stir together until cornstarch is completely absorbed.

Continue adding the remaining cup of cornstarch a little at a time until your mixture thickens. You're looking for your oobleck to feel hard when you tap on the surface, but your finger will sink into it if you press and hold.

You may or may not use all two cups of cornstarch. If you mixture starts to dry out, simply add more water. If your mixture is too runny, add a bit more cornstarch.