An old one but i'll still tell it.
Sign in pet shop window, 'talking dog £1.'
Bloke sees sign, goes into the shop, straight to the dog.
"Talk to me", the bloke says.
The dog looks up and says, "Well, I discovered my gift of talking pretty young and I wanted to help the government, so I told the MI5 about my gift, and in no time they had me jetting from country to country sitting in rooms with spies and world leaders, because no one figured a dog would be eavesdropping. I was one of their most valuable spies eight years running.
"The jetting around really tired me out, and I knew I wasn't getting any younger and wanted to settle down. So I signed up for a job at the airport to do some undercover security work, mostly wandering near suspicious characters and listening in. I uncovered some incredible dealings there and was awarded a batch of medals. Had a ife, a mess of puppies, and now I'm just retired."
So the bloke goes to the shopkeeper and asks why the dog is so cheap?
"Cus he's a liar" said the shopkeeper. "He ain't done any of those things"