I was thinking of it this week when interviewing applicants for a teaching post. We ask would-be teachers to conduct a lesson that we observe. We also get them to meet a panel of students who give us feedback. And in the interview itself we test candidates' subject knowledge (what do they know?) and their pedagogy (how would they teach it?).
We think this is important. Subject knowledge on its own is not enough to guarantee that a teacher will be any good. In fact, it can often be synonymous with them being bad. I suspect we have all been taught by a supposed expert who, as the 18th century poet Oliver Goldsmith put it, provoked our admiration "that one small head could carry all he knew".
We admired the depth of their knowledge, their endless capacity to perambulate around the outer edges of their subject and their ability to answer any obscure question. And their lessons bored us into a state of comatose submission.
Yes, we all realise that knowing a lot does not guarantee good teaching. As a feckless teenager I was taught by someone who clearly had a passion for his subject - religious education. Sadly, none of us shared it, chiefly because of him.
I would wait until five minutes into every lesson and ask a provocative question, such as: "What is God?" Then, like a child who has thrown a firecracker over a fence, I would sit back and watch the teacher assume we were interested in his labyrinthine responses. We weren't. For him, my question was a sign of our enthusiasm. For us, it was a mark of contempt.
I look back with distaste at this knowing and sadistic game of "bait the teacher". But it was also an early lesson about what great and not great teachers do and don't do.
This week at interview, a candidate told me that she would never talk to a class for more than 10 minutes because "Generation Y" children somehow needed to learn everything through computers.
I was unconvinced. In my experience, young people have so much ready access to information, with the most arcane nugget of knowledge retrievable in seconds from cyberspace, that they take it for granted. It holds few mysteries.
So when they do get a teacher who knows a lot but can also explain it in a way that is engaging, impassioned and exudes fascination, they love it.
Young people appreciate great teachers as much as they ever did but are more intolerant of mediocre ones. They know that time spent listening to an educational duffer could be more productively devoted to a search engine. But brilliant teachers don't just give us information. They give it a context, they share a perspective, they help to make sense of a world of giddying complexity, but without misleading by oversimplifying.
It's why as school leaders we need to keep our focus entirely on the classroom, on developing the next generation of inspiring teachers.
Our aim ought to be to prompt a student to say: "Why do I need Google when I've got a teacher?"
Geoff Barton is headteacher of King Edward VI School in Bury St Edmunds, England.
Original print headline: Let's just bait and seeWhat makes a good teacher?