One Smart Dog?

Two dog owners were bragging about the intelligence of their pets.

"The brightest dog I ever had," said one, "was a Great Dane that used to play cards. He was a whiz at poker, but finally a friend complained about him and I had him put to sleep."

"You had him put to sleep, a bright dog like that? A dog like that would be worth a million dollars."

"Had to", he replied, "Caught him using marked cards."