Alan, Bob, and Carl are three brothers who look so much alike that I can't tell them apart. I meet one of them on the street, and need to know if it is Bob, to whom I need to repay a loan. (I don't owe any money to the other two.)
I know that Alan always tells the truth, while Bob and Carl always lie. What one question can I ask the brother I meet on the street to determine if I owe him money?