Four logicians walk into a bar.
The barman asks “Does everyone want a drink?”
The first logician says, “I don’t know”.
The second logician says, “I don’t know”.
The third logician says,“I don’t know”.
The fourth logician says “Yes”.
If the first logician did not want a drink, she would immediately answer, "No." Since she doesn't do that, she must want a drink. However, she doesn't know what her 3 colleagues want, so she must reply, "I don't know."
The second logician wants a drink and realizes the first logician also wants one but, by the same analysis, #2 doesn't know what their other 2 colleagues want. So, she too replies, "I don't know."
Ditto for logician #3 who knows that #1, 2, and 3 want a drink but none of them knows #4's state of thirst.
Logician #4 knows she wants a drink and has reasoned that each of her 3 colleagues also wants one. She, therefore, confidently answers, "Yes" to the barman and orders 4 single malt Scotches, each. (It's her turn to buy for the logician's club annual outing.)