To get to the nitty gritty of this brain teaser, narrow your focus. Instead of looking at whole figures, pick a single body part only. Pick a man in the Start picture, look to see where your chosen body part is, and then look to see where it appears in the End configuration. Now repeat this for all 12 men. In all cases — and this is the key point — one of the 12 instances of a body part in the first picture is bisected and used twice in the second.
For example, let's look only at faces. Man #1's face in the Start picture is below the divider, so it remains with man #1 in the End picture; man #2's face (along with the rest of his head) goes to man #9; man #3's face goes to man #10. So far so good? If not, take it slower.
Now look at man #4. His face is split in half, with his top half going to man #11 and his bottom remaining with man #4. In other words, the single face owned by man #4 in the Start configuration is now two faces in the End configuration; in other other words, where there were 12 faces, there are now 13.
Pick another body part, do it again, and again. You'll see that one of the body parts in the first picture is split and used as two parts in the second.
Here's the breakdown:
* Hair: #1 is used on both #1 & #8
* Face: #4 > > > both #4 & #11
* Arms: #2 > > > both #2 & #9
* Torso: #9 > > > both #5 & #9
* Crotch (i.e., point where legs meet torso):
#5 > > > both #5 & #12
* Legs: #12 > > > both #7 & #12
* Feet: #10 > > > both #6 & #13
So in the End picture we get a new head of hair, a new face, a new pair of arms, a new torso, a new crotch, a new pair of legs, and a new pair of feet — all of which add up to an entire new person. But these parts are distributed among 13 different composites. Thus, you can't point to any one person in the second images and say "he's the new one."
Still not getting it?
To appreciate what's happening to the animated image (and to your eyes), click here.
Try each of these WCD brain teasers: