Lex Luthor's father advised his evil-genius son to invest in land because "it's the one thing they aren't making any more of". He was wrong. There's something even more precious and less malleable: time.
When it's gone, it's gone. The past is a country we are barred from re-entering. Logically, there is an opportunity cost with every moment that passes; what we could have done is the source of endless recriminations. If we waste a moment of a pupil's time - or allow them to do the same - we've frittered away a resource we can't replenish and they can't afford. And if they lack the second-chance safety net of wealth or connections, the misdemeanour becomes a felony.
So why are teachers made to sit like turkeys in a feeding pen in so much of what passes for staff training? I'm talking about Inset, that opportunity for a school to throw away every lesson they have learned about learning; where adults are forced to listen for hours without interruption. (And if you've never enjoyed an hour-long lecture about independent learning then you have no real concept of purgatory. And the lecturer has no concept of irony.)
All too often Inset resembles Bullshit Bingo, a game for minions of all ages: simply construct a bingo sheet consisting of the latest fashionable moronisms, tick them off and wait for someone to shout "house". I first encountered the game when I worked in middle management in a restaurant chain many years ago. You could forgive knuckleheaded platitudes there, but in a school? When did we take the word training and redefine it as "being instructed about the latest workload requirements"?
The irony is that teachers need to re-energise their practice constantly: the emotional demands of the job mean a pit stop, a gear change, a sabbatical or a power-up is necessary every five years or so if you want to avoid madness or, worse, entropy.
There cannot be many professions that need their batteries changing more frequently. But too many schools give their staff lead weights when they should be giving them rocket boots. Too many twilights are spent listening to dreary mission statements or leaden memos that should have been emailed, or attending training that is aimed at everyone and therefore no one.
Training should be, needs to be, so much more. Schools should be seeking to engage with teachers' needs as much as their own perceived requirements. If I never see another sheet of sugar paper it will be too soon. And the next time a jocular facilitator tells me that I should turn to the person next to me and tell them a secret - as if what teachers needed most were the gift of indiscretion - I will elbow the fire alarm on my way out.
Because CPD isn't something you do to people, especially badly. It's something that a human being does for his or herself. Headteachers should let staff play a part in deciding what they need. Watch as they take off. And then let them fly.
Tom Bennett teaches at the Jo Richardson Community School in Essex and is director of the ResearchED conference