Popcorn pops because ... you set your microwave to two minutes on high, right? As you might expect, there's a little more science to it than that, and two French engineers have dug into exactly what happens when a kernel of corn does its thing. Their formula-filled study, published in the Journal of the Royal Society Interface, kicks off with this sentence: "Popcorn is the funniest corn to cook, because it jumps and makes a 'pop' sound in our pans." But Emmanuel Virot and Alexandre Ponomarenko get far more scientific from there, noting that while previous studies have examined the conditions needed to properly pop corn, the physical origin of a kernel bursting open, jumping, and making that sound remain "elusive in the literature." Using elementary tools of thermodynamics and fracture mechanics (along with an oven, a microphone, and some cameras), they set out to solve the mystery.
Through a series of tests, they determined that at 356 degrees Fahrenheit, 96% of corn kernels are popped. But before that, at 180 degrees, some of the kernel's internal moisture (the Guardian reports kernels are 14% water) begins to turn to steam. At the 356-degree point, pressure has multiplied to roughly 10 bars; that's 10 times the atmosphere at sea level, explains Phys.org, and it causes the hull to fracture. A camera taking 2,900 frames per second captured what happens next: At 6.9 milliseconds, the starch expands; at 13.8ms, "we observe the formation of a 'leg' which is compressed on the plate." Some 7ms later, "this leg bounces and the popcorn jumps." As for the pop, they determined that it's not caused by the kernel cracking or the popcorn jumping (as in a pan)—it's triggered by the pressurized vapor being released. "More precisely, the pressure drop excites cavities inside the popcorn as if it were an acoustic resonator." (Scientists have also finally figured out how many licks it takes to get to the center of a Tootsie Pop.)