|> > There is an island,on which there are 100 blue eyed people and 100 brown
|> > eyed people. The people, however, do not know the number of people with
|> > each eye colouring. No-one knows their own eye colour, as there is no means
|> > of reflection, and no-one will tell anyone else their eye colour, since as
|> > soon as someone knows their own eye colour, they kill themselves 24 hours
|> > later.
|> > Once every 100 years, a green-eyed guru speaks to the populus, and today's
|> > the day (quite by chance). S/he stands on a rise, looking out over all these
|> > people (who don't know their own eyecolour), and announces: "I can see
|> > someone with blue eyes"
|> >
|> > Who kills themselves, and when???
Unfortunately, I have just joined the group so I did not catch the original article.
But the solution to the puzzle is ... <SPOILER>
Suppose there was just one blue-eyed person. This person can only see brown-eyed people.
If the guru says "I can see someone with blue eyes", this person knows immediately that s/he has blue eyes and kills himself on the next day (call this day 1 AG - "after guru").
All the other people (with brown eyes) can tell from the suicide of the blue-eyed person
that this person has not seen anybody else with blue eyes (otherwise s/he would not have been able to conclude that s/he has blue eyes and killed himself on day 1 AG). So all brown-eyed people know on day 1 AG that they have brown eyes and kill themselves on day 2 AG.
If there are two people with blue eyes, each of them can see one person with blue eyes.
If this person does not kill himself on day 1 AG (s. above), each of them knows that there must be someone else with blue eyes. Therefore each of them knows on day 1 AG that s/he
has blue eyes and kills himself on day 2 AG.
The brown-eyed people can tell from the double-suicide on day 2 AG that each blue-eyed person
has seen only one other blue eyed person. Thus they know on day 2 AG that they have brown eyes and kill themselves on day 3 AG.
...
If there are 100 blue-eyed people, each of them can see 99 people with blue eyes. If these people do not commit suicide on day 99 AG, there must be someone else with blue eyes. Thus every blue-eyed person knows his eye colour on day 99 AG and kills himself on day 100 AG.
The brown-eyed people can tell from the 100-fold suicide on day 100 AG that there were exactly 100 people with blue eyes and that they all have brown eyes. Therefore they commit collective suicide on day 101 AG.
Note: The number of people with brown eyes does not matter in this tragedy.
