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 any of the first three logicians didn't want a drink, they could have/would have answered no to the bartenders question, because they know that they themselves don't want a drink. If they want a drink themselves, they can only say "I don't know." The fourth logician deduces they all want a drink.