Riddle: You have been captured by pirates. They will only let you go if you do a task for them. If you fail then you will be killed. They show you a partially empty barrel of rum, and you have to tell them if it is more or less than half full. You have no tools, and you cannot let a drop of rum leave the barrel. How can you do it, and avoid walking the plank?
Answer: Tip the barrel so that the rum just reaches the rim. If you can see the bottom of the barrel it is less than half full, and if you can't it is more than half full.