«Letting the cat out of the bag» — is an idiom used to indicate that a secret has been revealed. Its origin likely dates back to the 18th century when market traders would deceive buyers by selling a cat in place of a piglet. Upon opening the bag at home, the buyer would discover the trick, hence the idiom's meaning. In modern usage, this phrase applies to any situation where a secret is unintentionally disclosed. For example, if someone accidentally mentions a surprise party in front of the guest of honor, you might say, «Well, the cat's out of the bag now!» [Русский аналог идиомы: «Всё тайное стало явным»].