Rice Crispies contain lots of sugar and are cooked at high temperatures. The sugar forms crystals and also creates air-filled pockets.
So, when Rice Crispies absorb milk, forces push the air which then breaks the cavity walls and make the snap, crackle and pop noise. As far as I know, this is the only food that does this.
And just in case you were wondering… no they are not a particularly healthy breakfast. Far too high in sugar and refined carbohydrates. You’d be much better off with a wholegrain cereal which will give you longer lasting energy along with some fresh fruit.