Edmund Harris mentioned on Twitter that a Slinky can make  a very good model of a Klein bottle. Elin Roberts asked for an explanation, and I was curious too, so I did a bit of rummaging and found my old Slinky. ((it isn’t a real slinky, but what’s the generic name for them? Something like ‘coiled metal extend-o-toy’?))

Once I’d worked out what to do, I recorded a video explaining it: