This is an animated version of a lovely old Victorian puzzle.  Basically, you have to count the number of people in the image, wait for the piece to flip around, and count them again.  The flip should happen after about 10 seconds (if it doesn’t, you might have to click on the image and have it open in a separate window)

My thanks to @DrBrocktagon for bringing it to my attention.  10 points to anyone who can explain why it works!

1. fergusg says:

Take 12 people and take a 1/12 slice from each, effectively to make 13 people, each with 11/12th of a body. Notice how each is at a different height so the slice passes though a different part of each. When in the “13” configuration, each one is slightly shorter – the guy on the left has the top of his head missing, the 2nd and 4th guys in the front row have oddly squashed faces, etc.

2. Is this how they do waiting list statistics?

1. Lazy T says:

Ah! I wondered why they had introduced a bacon slicer to Terminal 5.
It’s a very clever method.

3. tortorific says:

Look at the guy on the far right, when there are twelve he has longer legs then when there are 13, the next guy loses half his head, the next one half his shoes, the next one a chunk of his torso, as said above they are making everyone smaller so that there is an extra person. To make it work there has to be one person who’s whole body moves for the picture with 13 and that’s the third guy (the highest guy) in the second row from the left and there has to be someone who loses something and gets nothing when it goes to 13 and that’s the guy in the bottom left (or the first guy in the first row on the left).

4. Anonymous says:

Clue: Watch the guy on right on the first row, the line of his vest becomes a chin!

5. Duncan says:

Watch the guy in the front row on the far right. The line of his vest becomes a chin!

6. What appears to happen at first glance is that the people are divided in half and rearranged; but if you have the same number of heads and the same number of feet you should always have the same number of people, and you don’t, so where does this extra person come from?

The answer is, that’s not actually what’s happening. Look at the guy on the far left: when the top half is separated, he gets a haircut–a haircut he keeps after the people are rearranged. He’s the “extra person,” a bottom half that no longer gets a top half.

1. And the same is true of the guy third from the right: when divided, he loses the very bottom of his shoes and doesn’t get them back. So you have a bottom half and a top half that are now two extra people.

7. Anonymous says:

So cool

8. spiderabc1 says:

It was the guy on the left that I noticed first. Nice hypnotic illusion.

9. Dharmaruci says:

this puzzle can be greatly simplified. the row at the back changes from four men to five. the rest is distraction.

10. eedwardgrey69 says:

I can explain – but only because I had that one as a kid and spend a lot of time thinking on it…
(And when I said “I had this” I mean I had three strips of paper…. back in the days when we had things on paper….)

11. This the first time for me to see an electronic version of this trick. I love it. Mariano, I love your collection starting from 1880–very cool collection.

12. For many years the first thing I did when I was interviewing someone for a job was to show them a version of this, “The Vanishing Leprechaun”. I’d ask the job candidate to explain how it worked. If they could it was always a really bad, self-deceiving answer so I knew right away I didn’t want to work with them. If they were just totally confused and fascinated at the same time, the the job interview could continue.

(an image of the vanishing leprechaun is here: http://www.angelfire.com/hi3/pearly/humor/1lep.html)

1. Kudos to Pat Patterson Lyons, creator of “The Vanishing Leprechaun” (rarely properly credited).