My hardest one I've heard was this:
You have three bags of gold coins. One of the bags has fake gold, which will weigh a little more than the real gold, but you don't know which bag it is. There's a penny scale, but you only have one penny. How do you find out which bag has the fake gold?
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Look up deduction puzzles. Hardest shit I've ever tried to solve, rarely can. Here's one of the more basic ones: You come across a two doors guarded by two guards (one in front of each door), a sign says one guard only tell the truth the other only a lie. They can only tell you yes or no. One door will lead you where you must go, the other door leads to death. You can only ask one question of one of the guards. What should you ask the guard you choose?1