Here is a classic puzzle. I don't know its origin for sure, but I think Martin Gardiner might have published it in his long-standing Scientific American column.
I have three bags and I know one contains apples, one contains pears, and the third both apples and pears.
Each bag is labelled to say what its contents are but unfortunately the labels have got swapped round so all three are wrong.
I am allowed to choose one bag and take out a single fruit. I want to work out what the correct labelling of all the bags should be. How can I do it?