Is That How It's Done?

A colleague has just been reassembling some equipment and has held up two leftover screws:

'Where did these come from?'

It occurred to me that, unlike eukaryotic organisms, mechanical components tend to breed when you separate them!

Hmm! That's got the inspiratons whizzing: I now recall from somewhere a mathematical description of how to slice up an object and reassemble the pieces into two identical objects.