A popping popcorn kernel reaches 135 pounds per square inch of pressure, nearly four times the pressure inside a car tire.

Popcorn is the only variety of corn that pops reliably due to its unique hull structure. Each kernel contains a tiny droplet of water stored inside a circle of soft starch. As the kernel is heated to approximately 355 degrees Fahrenheit (180 degrees Celsius), the water turns into pressurized steam. This steam transforms the hard starch into a hot, gelatinous goop.The pericarp, or the outer shell of the popcorn kernel, is unusually thick and strong compared to other corn types. This allows it to act as a pressure vessel that can withstand internal forces of up to 135 pounds per square inch (psi). For comparison, most car tires are inflated to only 32 to 35 psi. When the hull finally fails, the pressurized steam expands rapidly into the lower-pressure air outside.Researchers at the French National Center for Scientific Research (CNRS) used high-speed cameras to study this process in detail. They found that the 'pop' sound is actually caused by the sudden release of pressurized air, similar to a champagne cork. Within milliseconds, the gelatinous starch expands, cools, and solidifies into a white foam that is 20 to 50 times larger than the original seed. This physical transformation is a perfect example of thermodynamics and material science in action.