Until this month I had never given much thought to the symbolism of the humble magnet. But a few weeks ago I was taking part in a discussion at the Chicago Forum on Global Cities. And there I got chatting to David Kilcullen, a former officer in the Australian army and now a military adviser in the US, who told me a curious tale that should make us all reflect on how we think about security.

Kilcullen explained that, until recently, American military engineers tended to design drones assuming that they would not crash. They welded the parts together, which meant that if the drone ever did crash, it broke in unpredictable — and irreparable — ways.

But these days the engineers are realising that they need to prepare for a worst-case scenario. So instead of welding the parts, they are starting to stick them together with powerful magnets. In this way, if the drone crashes, it will break up neatly — and can be salvaged and reassembled.