On an island in the middle of the ocean lives several people of a peculiar sort: They are all logicians. One hundred of them have blue eyes, while the others have brown eyes. The leader of the group, known as the guru, has green eyes. It so happens that no one on this island knows the color of their own eyes.
At midnight every night a ferry arrives at the island, and leaves shortly thereafter with anyone who can tell the ferryman their eye color.
One day, the guru stands up and says, so that everyone hears:
"I can see at least one person with blue eyes."
What will happen? Will anyone leave the island? Who, and when?
Please not that no one will tell anyone else their eye color, or communicate it in some other. As soon as someone knows their eye color she will leave the island with the next ferry - they all want to leave this weird place, after all. During any given day, all islanders will see each other, and notice any absences.
Related puzzles: Three hats
There are several variations of this puzzle, with different names, including Josephine's Problem and The Muddy Children Problem. The most popular version might be Blue Eyes by Randall Munroe.