hardLogicBy the Think Fanny puzzle desk

Blue eyes, brown eyes, and a ferry

Published July 14, 2026, last checked July 31, 2026. Re-solved cold against the published wording before it went live: how puzzles and answers are checked.

A famous induction puzzle. Why does the guru's sentence start a countdown? From the free puzzle set on Think Fanny. Next try Three switches, one bulb.

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The puzzle

On an island, there are 100 blue-eyed people, 100 brown-eyed people, and a guru with green eyes. Everyone can see everyone else’s eyes, but no mirrors and no talk about eye color. If a person ever becomes sure their own eyes are blue, they must leave on the midnight ferry that night.

They are all perfect logicians. For years, nobody leaves.

One day the guru says, in public: “I can see someone who has blue eyes.”

What happens, and when?

Where this puzzle comes from

Nothing in the answer is our invention. This is the standard common-knowledge induction problem, and the countdown is the documented part worth reading around:

If one of these contradicts the answer above, we would rather hear it than not. See how puzzles and answers are checked.