The police put up notices warning the public about a certain type of crime, but actually helped the criminals. How?
Clue: The police notices were to worn people about certain types of thieves. The thieves observed people’s reactions to the signs.
Answer: The police put up notices that read, “Beware of Pickpockets.” The pickpockets stood near a sign and noticed that when people saw it they immediately checked that their purses and wallets were safe. The pickpockets then knew where their victims carried their purses and wallets – which made them easier to steal.