This week: hand grenades
Some may argue that hand grenades (according to Wikipedia, any small bomb that can be thrown by hand) could have a place in the classroom. With a bit of imagination, all sorts of lessons could be livened up by studying explosive devices.
Some teachers even sigh fondly at Roger McGough's tongue- in-cheek poem The Lesson, in which a classroom of hooligans is schooled with a range of ordnance.
But the joke wears thin when the weapon is actually in the room, as happened in Australia this week after an 11-year-old brought in a "pineapple-shaped" money box for show-and-tell.
At Hunter Christian School in Newcastle, New South Wales, all 500 staff and students were evacuated to a park and allowed back into school only after the state police and bomb unit declared it safe.
Of course, some may argue that this demonstrates that not only hand grenades but also show-and-tell should be placed on the naughty step (if not violently shoved off it to be lost for ever in an eternal pit of hellfire). But that would mean depriving children of learning to sit quietly while classmates show them a pen, a picture of their cat or a toilet-roll tube that they've "made" into Lionel Messi. And the skill of being able to sit through irrelevant, dull and downright strange presentations is one that will be invaluable to them in future employment. So I'm afraid show-and-tell is in the clear - for now.