Dog

 A guy sees a sign in front of a house: "Talking Dog for Sale." He rings the bell and the owner tells him the dog is in the backyard. The guy goes into the backyard and sees a black mutt just sitting there.

-"You talk?" he asks.
-"Yep," the mutt replies.
-"So, what's your story?"
The mutt looks up and says, "Well, I discovered my gift of talking pretty young and I wanted to help the government, so I told the CIA 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 I 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 wife, a mess of puppies, and now I'm just retired."
-The guy is amazed. He goes back in and asks the owner what he wants for the dog. The owner says, "Ten dollars." The guy says, "This dog is amazing. Why on earth are you selling him, so cheap?"
The owner replies, "He's just a big liar. He didn't do any of that stuff."