A group of people with assorted eye colors (say 10 blue and 10 brown) live on an island.
No one knows the color of their eyes as there are no mirrors on the island and the water is muddy so you can't use the reflection. For all each person knows, they could have green eyes!
However, everyone can see the eye colour of everyone else, but they can't communicate to each other to tell each other their eye colour.
Every night at midnight, a ferry stops at the island. Any islanders who have figured out the color of their own eyes can leave the island.
One day a sailor from the ferry gets off the boat and says:
"I can see someone who has blue eyes".
Everyone hears and understand the statement, but the sailor is immediately shot dead for communicating with the islanders and no-one ever speaks again. However, given this information some people are able to figure out their eye colour.
Who leaves the island, and on what night?
It's difficult, but possible.
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