I'm sure this has been shown here over time, but I thought I'd show you one of my balloon breaking pictures. An old friend of mine and I did this in the 70s and I wanted to show my daughter how we did it then. I guess it's one of the safer experiments that I can reenact from the 70s! 
I made a cheap, sensitive, electrical switch and attached it to a hot shoe. My 530EX flash was connected to the hotshoe, seperate from the camera, set on manual mode and on 1/16th power. The camera was on a tripod in bulb, f/11, ISO 100. I ran a thread from an anchor point, past the back of the balloon, to the switch, taping the thread to the balloon. Camera on bulb, flash armed, lights off, shutter depressed and held, pull trigger on BB gun, BB hits balloon, balloon starts to implode, moves thread, shorts out switch, fires flash, recorded in camera, shutter released, lights on, repeat.
I smudged out the black thread in the images.
And, yes, I know I can buy a sound activated switch. I guess that's next.
There is still a blur in the BB. Perhaps I can decrease the flash fraction and then pump it up in post processing.
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