According to The Wiki, the manufacture of Pop Rocks is similar to what happens inside a volcano. The ingredients are heated until molten and then exposed to pressurized carbon dioxide at 600 ppsi. The gas is trapped in the viscous mixture of sugars, to be released at a pressure of 40 atm when these sugars encounter saliva. (Did someone really measure the pressure inside a Pop Rock? How do you do that without puncturing it?)
Are Pop Rocks dangerous? There's the story of Mikey (the kid who liked it, may he rest in peace), who ate them with cola. Reports vary but either his stomach or mouth exploded, and he died. In 1989 I watched "Ghost Busters II" in the theater and when the painting of Vigo started talking, I started, spilling my pop rocks into my shorts (possibly culottes)--painful, but not deadly.
As practitioners of the scientific method, the Wind-Up Bloggers knew we had to move beyond this kind of anecdotal evidence. Hypothesis: mixing Pop Rocks with a carbonated beverage should cause no harm. Method: subject placed ~.5 gram pellet of green (watermelon flavor) pop rock into mouth containing a small sip of vinho verde, a cheap, lightly carbonated summer wine. Results: subject reported a tingling sensation. Popping sounds of increasing volume. Conclusion: Pop Rocks are not dangerous and mixing them with alcohol is a good idea. They are even better with some cheap sweetish red wine (more intense pops). Mixing pop rocks and a sip of beer is not too thrilling, probably because the beer's own, more aggressive carbonation masks the popping rocks.
Thanks to flickr user Jason Michael for image of Pop Rocks on a tanning bed (?!).