A man takes his Rottweiler to the vet.
"My dog's sick" he says. "Is there anything you can do for him?"
"Well," says the vet, "let's have a look at him." So he picks the dog up and examines his eyes, then checks his teeth. Finally, he says, "I'm going to have to put him down."
"Why? Says the man in alarm.
"Because he's bloody heavy!"
This joke makes use of the double meaning of to put down. If you put something down, it means you stop holding it. However, to put a dog or other animal down, it means to kill it.
If you think this joke is bad, just wait till next week's!