It's morning time, the birds are singing, the sun is shining, and your breakfast cereal is making a racket.
Those crispy, puffed up bits of rice were quiet enough in their box, but pour your milk, hold your ear close, and your breakfast sounds like Chinese new year. Why do crisped rice cereals make so much noise?
During the cereal making process, the cereal grains are dried and cooled. When almost any material cools or dries out, it tends to slightly change its size, shrinking a little bit here, a little bit there. In something as complex as a starchy piece of cereal, this shrinking happens unevenly, and stresses are created in the brittle material.
If you live in an old house, you can probably hear the same kind of thing happening when it cools down at night. The floorboards creak and pop as cooling shrinks them unevenly. Drying can do the same kind of thing. The cracked surface of dried-out mud is a record of the stresses created when the mud was drying.
When your cereal was cooled and dried in the factory, it didn't pop or crack right there. Instead, those stresses were locked in place in the rigid cereal grains, like a million rubber bands stretched tight and waiting to snap. When you pour your milk on them, the fireworks begin.
As milk touches one side of a cereal grain, it swells suddenly and distorts its shape. This is too much for the stressed-out dry part to take. It snaps, crackles, and pops, as those stored up stresses crack the cereal.