From LiveScience.com:
A Rice Krispie behaves like a piece of glass. If you hit it hard, it’ll break into a puzzle of a million pieces that, if you had the patience, you could put back together again.
The high temperature at which the cereal’s cooked creates extremely strong bonds that hold the rice’s starch molecules together. The strong bonds make the rice act like glass.
During the cooking process, each piece of rice expands and a network of air-filled caves and tunnels form inside.
When you pour milk into your breakfast bowl, the cereal absorbs the milk. As milk flows into the crispy kernel, the liquid puts pressure on the air inside and pushes it around.
The air shoves against each pocket’s walls until they shatter, forcing out a snap, or a crackle, or, as you, know, sometimes a pop.
