15 Prisoners – Solution

The prisoner puzzle is apparently annoying other people as much as it did me, which is, you know, gratifying. It was a present when I was a kid, and became one of those items that pops up intermittently over the years, annoying me more each time. I confess I never figured it out, and I eventually stumbled across an explanation online. There are a few different versions out there, all using the same principle.

So, what’s happening to the extra prisoner? He’s being absorbed into all the others.

Look carefully at the remaining prisoners and you’ll see that each gains a small amount of mass – a bit of leg here, a forehead there. The puzzle is cleverly designed so that the individual prisoners appear to remain consistent between turns, but actually each body shape is duplicated, so they’re not ‘the same’ people after the turn. But, the duplicates are slightly different sizes. Check out Bushy Eyebrow guy just next to the ‘A’, going clockwise. When the wheel is at ‘B’ his legs are just that much longer than at ‘A’, and indeed all of the prisoners are slightly bigger. The extra guy is being chopped up into lots of little pieces and distributed amongst the rest. Cannibal prisoners.

Still don’t like it. I can understand it intellectually, just about, but I can’t grasp it properly. Writing this out has helped a little, but I still intuitively dislike that I can’t ‘see’ the guy disappear. It’s a good one, though – I enjoy breaking my brain.

If my explanation hasn’t helped, there’s a particularly clear description here, and moillusions has further examples.