Idiom of the day :- Let the cat out of the Bag

Usage, examples and definition of the Idiom :-

Bob was going to retire from teaching in June, and the foreign language department was planning on presenting him with some luggage at his retirement dinner. He wasn’t supposed to know about it, but someone let the cat out of the bag. At the dinner Bob acted surprised, even though someone had told him what he was getting before the official presentation.