A man walks into a bar and asks the bartender, "If I show you a really good trick, will you give me a free drink?" The bartender considers it, then agrees. The man reaches into his pocket and pulls out a tiny rat. He reaches into his other pocket and pulls out a tiny piano. The rat stretches, cracks his knuckles, and proceeds to play the blues.
After the man finished his drink, he asked the bartender, "If I show you an even better trick, will you give me free drinks for the rest of the evening?" The bartender agrees, thinking that no trick could possibly be better than the first. The rat again starts to pound out the blues on the little piano. The man reaches into another pocket and pulls out a small bullfrog, who begins to sing along with the rat's music.
While the man is enjoying his beverages, a stranger confronts him and offers him $100,000.00 for the bullfrog. "Sorry," the man replies, "he's not for sale." The stranger increases the offer to $250,000.00 cash up front. "No," he insists, "he's not for sale." The stranger again increases the offer, this time to $500,000.00 cash. The man finally agrees, and turns the frog over to the stranger in exchange for the money.
"Are you insane?" the bartender demanded. "That frog could have been worth millions to you, and you let him go for a mere $500,000!" "Don't worry about it." the man answered. "The frog was really nothing special. You see, the rat's a ventriloquist."
My sister's been telling this one forever. Its still funny.