An honest person has three stones, one white, one grey and one black.
They secretly chooses one stone, and allows you to ask one question, to which they will answer “Yes”, “No” or “I don’t know”.
Can you determine which stone the person chose?
Ask “If I chose a stone out of the two that you didn’t pick, would it be darker than yours?”
- If they have a black stone, they answer “No”.
- If they have a white stones, they answer “Yes”.
- If they have a grey stone, they must say “I don’t know”.