The fluffy white part of popcorn is actually starch that instantly turns from a liquid into a solid as it pops.

Popcorn is the only variety of corn that pops reliably due to its unique hull thickness and starch composition. Each kernel contains a tiny droplet of water stored inside a circle of soft starch, which is surrounded by a hard outer shell called the pericarp. As the kernel heats up to approximately 355 degrees Fahrenheit (180 degrees Celsius), the internal pressure builds to about 135 pounds per square inch. At this critical temperature, the moisture inside turns into superheated steam, which gelatinizes the hard starch into a hot, jelly-like goo. The pericarp acts as a pressure vessel until it can no longer withstand the internal force and ruptures suddenly. This rapid depressurization causes the gelatinized starch to expand into a bubbly foam that can grow up to 40 to 50 times its original size. As the steam escapes and the starch encounters the cooler surrounding air, it undergoes a phase transition. The liquid foam dehydrates and solidifies almost instantly into the white, crispy structure known as a flake. Research by physicists at the French National Center for Scientific Research has even used high-speed cameras to show that this process happens in just a few hundredths of a second, creating the signature 'pop' sound as the pressurized air is released.