When covering colleagues' lessons, any teacher, even the conscientious one, will often leave their brain at the door. Time for the old faithful reading lesson or some pointless copying task, which allows you to either snooze or craftily finish that tricky su doku.
But the other day I was presented with a profound moral question which has since kept me awake. The cover work was an unashamed hour-waster: "write five things which are good, five which are evil - give reasons".
No problem there, I thought, set the work, grabbed my su doku and settled in for a peaceful hour.
As I laboured, I could hear the comforting consensus of mumblings from the class. Chocolate: good. Homework: evil. Alton Towers: good. Dinner lady with wooden leg: evil. No surprises. Then, a dissenting voice. "Sir, I can't think of anything."
With a heavy heart, I laid my puzzle down. "Come on, 'good', you know: fluffy bunnies, Christmas, Gandhi, gravy, free stationery, Britney, tax rebates, ice cream. Evil: war, disease, housework, Blair, walnut whips, child abusers, cover lessons, it's easy."
He thought. In a film with Macaulay Culkin, smoke would have plumed from his ears - then the light bulb.
"Got it Sir! Bill Oddie, Sir."
"Bill Oddie, Sir. He's evil."
I tried to reason with him. "Really? The bloke off the telly? With the beard?"
"Yes, Sir. Him. Evil, Sir."
I sniffed the air for the tell-tale smell of marijuana. Nothing. No empty beer cans lying about, either.
I joked. No, he's not evil. He's one of the Goodies! My retro joke lost on this 12-year-old who probably thinks Vicky Pollard is comedy history now Catherine Tate has come along.
"No, Sir. Not a Goodie. One of the baddies."
Did he hate birdwatching? Or just men in Barbour jackets lurking in bushes with binoculars?
The boy went back to his list, but I was troubled. Could the genial ex-Goodie TV-presenting birdwatcher be a bad 'un? Perhaps his family and Oddie's had been locked in a blood feud for generations. Or was it the beard?
Intrigued, I inspected the class's work. When I reached the Oddie-hater I peered over his shoulder and saw the words neatly inscribed in the evil column: bin Laden (say it fast in a noisy classroom, with a CockneyBengali accent to a hungover teacher, see?) So, Oddie isn't evil. But it still made me think how evil isn't as absolute as we expect. I was prepared to think that the hapless Bill - one of the nicest blokes on TV by all accounts - was as bad as Osama. True, no major acts of terrorism to speak of. But a simple cover lesson had made me think how communication - or the lack of it - can affect our view of the world entirely.
Just to be safe, the next day I shaved my beard off.
Julian Kaufman teaches in a secondary school in east London