I guess - never having tried the party balloon trick myself - that such a small amount of hydrogen will mix well enough with the surrounding air as the balloon

I guess - never having tried the party balloon trick myself - that such a small amount of hydrogen will mix well enough with the surrounding air as the balloon pops to create a (modest, at least) bang as the mix ignites, but this is just conjecture. Maybe I'll have to do some empi research one of these days - we do have a few bottles of oxygen and acetylene in the workshop at my current employer. Hey ho, time for an engineer to pay our welders a visit.