You let the cat out of the bag when you tell a big secret. Example: "Bob didn't want anyone to know he was sick, but his wife let the cat out of the bag."

A secret is like a cat that has been kept a bag. Once you let the cat out of the bag, it goes wherever it wants. Example: "We were going to keep our plans to get married a family secret, but my mother let the cat out of the bag."

The person who told the secret is the one who let the cat out of the bag. Example: "Hey! Who let the cat out of the bag?" Example: "Oops. It looks like I let the cat out of the bag."