c4

The explosive C4, a favorite for everything from demolition to terrorism to action movies, is in fact one of the safest explosives. How can an explosive be safe? If it’s hard to set off by accident. C4 is so stable that you can light it with a match (it burns but does not explode) or shoot it (it splatters but does not explode). To go bang, it requires a detonator that produces both heat and shock.

At the other end of the spectrum are mixtures that ignite simply from being scratched or knocked. There are obvious challenges in mixing, storing, and handling these substances so that they explode only when intended, yet they’re surprisingly common.