You have ten bags of coins. Nine of them contain genuine coins but one of them is full of counterfeits. You cannot see any difference between the coins or bags, or feel any difference when you lift them. However, you know that each of the counterfeit coins weighs one gram less than a real coin. You have an accurate scale, but are only allowed to use it once. How do you work out which bag has the fake money in it?
Good luck -- it had me properly fooled, and yet the answer is so-o-o logical