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).

It’s a fine example of a so-called “non-Newtonian fluid.” Isaac Newton described the properties of an “ideal liquid” back in the 17th century, one of which is viscosity, loosely defined as how much friction/resistance there is to flow in a given substance. Oobleck is the opposite of an ideal liquid. As I wrote back in 2007:

The friction arises because a flowing liquid is essentially a series of layers sliding past one another. The faster one layer slides over another, the more resistance there is, and the slower one layer slides over another, the less resistance there is. Anyone who’s ever stuck their arm out of the window of a moving car can attest that there is more air resistance the faster the car is moving (air is technically a fluid).

That’s the basic principle. But the world is not an ideal place.... In Newton’s ideal fluid, the viscosity is largely dependent on temperature and pressure: water will continue to flow — i.e., act like water — regardless of other forces acting upon it, such as being stirred or mixed. In a non-Newtonian fluid [like oobleck], the viscosity changes in response to an applied strain or shearing force, thereby straddling the boundary between liquid and solid behavior.

Let the eleventh Doctor Who, Matt Smith, demonstrate:

That’s the big-picture explanation, but physicists have been arguing for decades about precisely what’s going on at the small scale, because experiments and computer modeling studies haven’t agreed. From a physics standpoint, we’re talking about microscopic particles suspended in liquid (colloids). On one side are physicists who think that friction between those suspended micro particles lock them in place to resist flowing like a liquid.