Who Let The Cat Out Of The Bag
The origins of this phrase come from when farmers would sell chickens at market. Most of the time they would just throw the chickens in a bag and sell them as however many were supposidly in there. But cats were cheaper than chickens so many times farmers would put a cat into the bag with the chickens and if someone suspected something they would open the bag. At that point the cat would jump out of the bag, thus giving away the secret.

This from a website visitor:
Eventually the duped customers would open their burlap bags revealing the cats and thus "letting the cat out of the bag". This expression is often used when the real truth has been revealed.





(来源：英语麦当劳－英语学习门户 http://www.EnglishCN.com)