Favorite logical puzzles

This topic is a bit related to my JavaScript interview questions post, because I think it is a very good test to see how a candidate relates to a so called 'difficult' problem. If she just freezes, it is a bad sign, but if she starts to cover the options, that's a very good sign.
Here are few of my favorite puzzles:

• You work at a fruit factory. There are 3 crates in front of you. One crate contains only apples. One crate contains only oranges. The other crate contains both apples and oranges. And each crate is labeled. One reads "apples", one reads "oranges", and one reads "apples and oranges". But the labeling machine has gone crazy and is now labeling all boxes incorrectly. If you can only take out and look at just one of the pieces of fruit from just one of the crates, how can you label ALL of the crates correctly?
• https://en.wikipedia.org/wiki/Prisoners_and_hats_puzzle 
• https://en.wikipedia.org/wiki/Thinking_outside_the_box (9 dots with 4 lines)
• http://www.smart-kit.com/s337/burning-rope-logic-puzzle-difficult/ 
• http://tierneylab.blogs.nytimes.com/2009/03/30/solution-to-100-hat-puzzle/?_r=0