what makes those rocks in the party poppers pop?

i know its some kind of reactant but what?

The secret ingredient is silver fulminate, according to this Wikipedia article:


Burf3 years ago

Snap rocks are gravel impregnated with a minute quantity of silver fulminate (AgCNO). Silver fulminate is a percussive that detonates under impact. Snap rocks are considered safe because the mass of the percussive is miniscule in comparison to that of the mass of the gravel.