"The talking dog" joke

A man walks into a bar with his dog and puts the dog on a barstool. The bartender asks the man what he wants to drink.
"I'll have a bourbon and Coke!"
The man then turns to his dog and asks, "What are you going to have, Rover?"
"I'll have a Scotch and soda - light on the soda," says Rover.
The bartender is skeptical about the dog talking. "Come on," he says, "that dog can't talk - you're a ventriloquist!"
"No, Rover can really talk! While I am in the restroom, you can have a conversation with him yourself - but don't let him out of your sight. He is a very valuable dog."
The man goes to the restroom. When he returns, the dog is gone.
"Hey, where's my dog? I told you not to let him out of your sight."
"Aw, I didn't believe that Rover could talk, so I gave him a quarter and sent him to the drug store to buy me a paper."
"Let's go look for him," said the man.
The two went to the drugstore - no Rover. They walked up and down nearby alleys and streets - no dog! Finally, they found Rover in an alley on top of another dog, pumping away.
Pointing his finger at Rover, the man says angrily, "How come you are doing this? You have never done this before!"
"First time I ever had any money!"