let the cat out of the bag

The cat's out of the bag!
The truth is revealed.
The truth is out.
Нам / мне все известно!

let the cat out of the bag — проболтаться, проговориться

Example 1: I'm sorry I've let the cat out of the bag about your surprise party.

Example 2: Marsha let the cat out of the bag. So we know all about the surprise trip to France.

A dishonest farmer, claiming to be selling a young pig, might substitute a cat or some other valueless animal in a tied bag. A circumspect buyer would examine the purchase on the spot; an unwary one would not do so until it was too late. Either way the cat would then be let out of the bag and the truth would be known.
To be sold a pup (=swindled) is a variant of this expression; perhaps some tricksters used a dog instead of a cat. A [pig in a poke] is obviously related; poke is an old word for a small sack and the whole expression means 'something bought or received without prior examination or knowledge'.

See also [spill the beans]