Mixing corn starch and water makes for a crowd-pleasing staple of science demos. The resulting substance looks like a liquid, but hardens instantly when you punch it—in fact, it’s possible to run across a pool of the stuff. Now physicists think they’ve figured out just what’s going on when this unusual material switches from a liquid to a solid, hopefully ending a long-running debate. They described their findings in a new paper in Physical Review Letters.
The mixture goes by many names, but the most common one is “oobleck,” a nod to the classic Dr. Seuss story, Bartholomew and the Oobleck. It’s really simple to make your own. Just combine two cups of corn starch—add three drops of green food coloring if you want that snot-like Seuss effect—and gradually mix in enough water to create a mixture that resembles pancake batter (usually about equal parts starch and water).