If you mix cornstarch and water in the right proportions, you get something that seems not-quite-liquid but also not-quite-solid. Oobleck flows and settles like a liquid when untouched but stiffens when you try to pick it up or stir it with a spoon. The properties of oobleck and other non-Newtonian fluids—including Silly Putty, quicksand, paint, and yogurt—change under stress or pressure, and scientists have long struggled to prove exactly why.