My first teaching job was at an old, prestigious boys' boarding school in Australia. (When I say old, it was old for Australia.) Every boy, whether a boarder or day-boy, and every staff member received a hot lunch in the dining halls each day. There were two dining halls: most of the boys at in the less ornate one, but the oldest boys and the staff ate in the more formal dining hall. Portraits of all the previous headmasters gazed down on us as we ate, except for the headmaster who had been sacked after less than a year for having affairs with a few too many boys' mothers.

I was teaching a topic on ecology, which required taking the class out into the school grounds to count the number and diversity of species in an ecosystem. The school grounds were extensive; there were about two dozen playing fields, a small farm for teaching Agriculture (which is an actual, examined subject in some Australian secondary schools), and a lot of bushland.

In search of a suitable spot for the lesson, I headed off down one of the paths through the bush that went to the various boarding houses, and soon found a peculiar tree. It had few branches, and few leaves, but an enormous trunk: it was old and close to death. What made it peculiar, however, was the hundreds of knives sticking out of it - clearly pilfered from the dining halls and thrown by bored schoolboys.

When I returned to the science department, I told my colleagues what I had discovered. One of them was an old boy of the school, and another lived in one of the boarding houses, and yet none of them had any idea about the knife tree.