Why does popcorn pop?
The Real Answer
Popcorn pops because a popcorn kernel traps a small amount of water inside a hard outer hull. As the kernel heats up, that water turns into vapor and the pressure inside rises while the starch softens.
Eventually the hull can no longer hold the pressure and ruptures. The sudden pressure drop lets the hot water vapor expand rapidly, pushing the softened starch outward. The starch then cools and sets into the light, foamy structure we recognize as popcorn.
Popcorn is a type of corn whose kernel structure and moisture are especially suited to this process. The pop is therefore a tiny pressure-driven expansion, not the kernel simply splitting open and revealing fluffy material that was already sitting inside it.
You hear the popping slow down and immediately lean toward the microwave, convinced you can identify the exact safe second by ear. Then you wait for one last kernel and burn the whole bag for it.
Why People Ask This
People ask because the transformation is so disproportionate: a small, hard kernel suddenly becomes a large, white, irregular piece of food. The visible explosion makes it natural to wonder what was inside the kernel and what the heat actually changed.