3 ms·

There's a good Numberphile video explaining this illusion: https://www.youtube.com/watch?v=cE44nr4d3iY https://www.youtube.com/watch?v=cE44nr4d3iY (Hint: loo

There's a good Numberphile video explaining this illusion: https://www.youtube.com/watch?v=cE44nr4d3iY https://www.youtube.com/watch?v=cE44nr4d3iY (Hint: look at the heads)

rhn_mk1 5y ago
This one includes flags as an easy hint.
- kazinator 5y ago
  The flags don't really "do" anything. In the one configuration, every flag is held by a boy. In the other configuration one boy has no flag, and there is a severed hand holding a flag.
  - rhn_mk1 5y ago
    That's actually wrong for the other configuration. In one, every boy has a flag. In the other, there's one boy with no flag, and 12 boys with flags. The appearance of a boy with no flags is an insight.
    - kazinator 5y ago
      None of the flags straddle the boundary between the two discs, so it is obvious that no trick is present affecting their apparent count: there are always 13. The diagram's inner disc contains one complete torso with arms and hands, which holds two flags in either configuration, so "every boy has a flag" is only true if you actually mean "has at least one flag", not if you mean "has one flag". Since the number of flags is not affected by configuration, one boy not having a flag is unsurprising. There are always 13 flags, so if there are apparently 13 boys, and one is still holding two flags, then someone must necessarily not have a flag. (In the A configuration, there are actually two boys with no flags. One clutches his head with both hands, holding no flag, and the other, next to the A, is flanked by a floating, detached hand which holds a flag. Therefore, that boy has no hand, and no flag. This severed does not seem essential, either; I believe the picture could be somehow repaired so that the detached hand is reattached. Then in the A configuration would be exactly one boy with no flag, as you say.)
      - rhn_mk1 5y ago
        > it is obvious that no trick is present affecting their apparent count I see you found the insight from the hint: flags are governed by different rules than boys.

There's a good Numberphile video explaining this illusion: https://www.youtube.com/watch?v=cE44nr4d3iY https://www.youtube.com/watch?v=cE44nr4d3iY (Hint: look at the heads)

rhn_mk1 5y ago

This one includes flags as an easy hint.

kazinator 5y ago
The flags don't really "do" anything. In the one configuration, every flag is held by a boy. In the other configuration one boy has no flag, and there is a severed hand holding a flag.
- rhn_mk1 5y ago
  That's actually wrong for the other configuration. In one, every boy has a flag. In the other, there's one boy with no flag, and 12 boys with flags. The appearance of a boy with no flags is an insight.
  - kazinator 5y ago
    None of the flags straddle the boundary between the two discs, so it is obvious that no trick is present affecting their apparent count: there are always 13. The diagram's inner disc contains one complete torso with arms and hands, which holds two flags in either configuration, so "every boy has a flag" is only true if you actually mean "has at least one flag", not if you mean "has one flag". Since the number of flags is not affected by configuration, one boy not having a flag is unsurprising. There are always 13 flags, so if there are apparently 13 boys, and one is still holding two flags, then someone must necessarily not have a flag. (In the A configuration, there are actually two boys with no flags. One clutches his head with both hands, holding no flag, and the other, next to the A, is flanked by a floating, detached hand which holds a flag. Therefore, that boy has no hand, and no flag. This severed does not seem essential, either; I believe the picture could be somehow repaired so that the detached hand is reattached. Then in the A configuration would be exactly one boy with no flag, as you say.)
    - rhn_mk1 5y ago
      > it is obvious that no trick is present affecting their apparent count I see you found the insight from the hint: flags are governed by different rules than boys.