Zinc

It is the high school ball on Saturday. John has to Arrive.  There is a tradition that one's arrival is in itself an Event, and the whole town gathers in a huge circle round the school entrance to watch the imaginative forms of transport - mostly American cars from 1957, but the fire service is usually pressed into bringing three teenagers in voluminous dresses and somebody always arrives in a Willys Jeep - and to applaud anything especially striking.  Last year John arrived on a penny farthing. This year it has to be something else.
"You could go on a recumbent."
"I did think about that. But everyone knows I've got one."
"What you really want is a human powered helicopter. The one that just won that prize -"
"How long did he fly?"
"Don't know. But he got three metres up in the air."
"It's a remarkably useless achievement. It's not as if he went anywhere. He was inside a shed. You could just get a ladder."
"All human powered flight is useless. What they really ought to do is get a hydrogen blimp and fit a pedalled propeller. That would actually be useful."
"What for? Where would you go in a pedalled balloon?"
"Well, um, you could go to Adele Island and back."
"Ever heard of a boat?"
"Yes but then you could just wheel it out of the shed and fly there. Actually what you want is a pedalled aeroplane with short thick wings and a thick fuselage all filled with hydrogen. Everyone's using carbon fibre to save weight, and hydrogen is just another sort of stuff, and I can't see why you shouldn't use one sort of stuff to lighten the load when they're already using another sort of stuff to lighten the load."
"How useful would that be? I suppose if you were out on a walk and you suddenly came across a big lake filled with sharks and piranhas you might be able to use it but it'd be a big thing to have to carry around just on the offchance. "
"Actually, what you really want is a hydrogen generator and a big bag. - Actually, what you really want is a stick of zinc and some hydrochloric acid."
"And you could get the hydrochloric acid out of your stomach just by puking."
"You could use a balloon."
"Or condoms."
"So all you want is a stick of zinc. And some condoms."
"You'd want a lot of condoms."
"Yeah but they inflate. And let's face it, any young man out on a walk is going to have pocketsful of condoms, just in case he meets a busload of schoolgirls desperate for sex."
"So all you want, then, is to go on a walk with a stick of zinc. Sorted. - How did we get into this conversation?"