If you dropped it in the middle of a crowd of your friends, it would jump four or five feet each time a charge exploded, and the direction - which could be sideways or straight upwards - was wholly unpredictable.
Now, as fans of period drama will know, a 1960s British railway carriage had a corridor down the side which ran past a series of compartments, each with six seats and separated from the corridor by a door with a glass panel. That meant that you could view the occupants as you walked down the corridor - rather as you might view reptiles at a zoo.
Those readers who are teenage boys will already have spotted the opportunity. Light your Jumping Jack, throw it into the compartment, close the door and jam it shut with your foot. Magic! You could watch through the glass as your friends fell over each other scrambling to get out of the way. Make it three jumping jacks - better still.
Unfortunately, it all got a little out of hand. Some compartments contained members of the public as well as boys. One boy had an air rifle. Anyway 50 police boarded the train at Reading and it was then that the real disadvantage of a boarding education became apparent.
Every boy had his name marked on each piece of his clothing so that you could say "John Smith" all you liked but it was your real name that went down in the officer's notebook.
The school beat 200 boys next morning, the youngest and strongest masters being deployed for the task.
Now I do not believe that a single boy suffered any damage, apart from the immediate pain, from that punishment. It was open, it was regarded by the boys as fair and, once it had been administered, it was over.
Of course, beating in schools has now long disappeared and no one is suggesting that it be reintroduced, but this tale makes a useful point. Sometimes it was an appropriate remedy: sometimes it was not ... it all depended on the circumstances.
The Children's Commissioner for England has suggested that we should follow New Zealand in banning the smacking of children rather than, as now, allowing parents to use reasonable punishment which does not amount to bodily harm.
How does one strike the balance? It must depend, mustn't it, on how important the right to smack children really is.
It is all very well in a nice middle-class family where the parents support each other and badly behaved children can be sent to another room. No one really needs to beat anybody there and whether smacking is outlawed is really academic.
It is a different game, however, for a solo mother in a damp flat with the noise coming down through the badly insulated roof and the money too short to provide a decent diet.
So a child adds to the pressure by winding her up and she clouts it to teach it not to do it again. If she didn't, she could be driven to something much worse.
Are we really going to forbid her a natural action? Are we going to say that she is a criminal because she cannot live as we do? Surely we should have more pity than that.
The moral middle classes dictating behaviour to their social inferiors belongs in the pages of Charles Dickens, not to 21st century Britain.
But what if we don't change the rules - and so far none of the political parties have joined the commissioner's bandwagon - what then?
Well, it will be a question for juries and magistrates to decide whether the punishment was reasonable, with a good margin for them to exercise discretion and mercy in difficult cases.
Not such a bad place to end up, you might think.
* Before retiring, John Watson was a partner in an international law firm. He now writes from Islington, London.
