What makes those rocks in the party poppers pop?
black powder and a friction igniter - somewhat like a match in a fold of sand paper although the sand paper is on the string and the "match" is on the paper tube.
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Or they may mean what used to be called "space rocks" or "space dust" when I was a kid, the candy that pops and bursts on your tongue.
Like this: https://www.instructables.com/id/Pop-Rocks/