The makers of Rice Krispies cereal get this question a lot. The answer lies in the toasting process: heat puffs up (expands) the rice kernels and stretching them. This process forms the very thin walls with Rice Krispies structure. When you pour milk on the cereal, thin walls fracture under the increased stress. As a result, there is that familiar—and fun—“snap, crackle, and pop” sound.