What is your favorite logic question, puzzle, or game?

What are some great questions, puzzles, or games that are solved logically? Have you ever completed them? How long did it take you? Have you ever gotten extremely frustrated over one?

Questions can be a simple one liner or a more elaborate one.
Puzzles can be wooden, metal, or even online versions.
Games can be video or board.

Please don’t provide the answer to your question or puzzle. Let someone else try to.