Flash triggered by the balloon popping, using an iPhone app called triggertrap.
45 Mins setting up, 1/1000 sec to make a mess on the floor :)

Setup:
Big reflector as background, using black side.
Balloon hanging from the end of a reflector holder arm
Flash camera right in small softbox (to reduce hotspot)
Small silver reflector camera left hanging from same boom arm as balloon.
Balloon about 2/3 filled with water and some more air blown in so it makes a good loud pop to set off the trigger.
I child's paddling pool underneath in vain attempt to catch the water