Not a pun, but reminds me of an old joke.
A guy hears a knocking at his front door. He answers it but nobody is there. He looks down and sees a small snail, which he picks up and flings into the garden.
Six months later he hears a knocking at the door. He answers it but nobody is there. He looks down and sees the small snail. The snail says "what the h*ll did you do that for?"