The Snail Joke (page 16)

Every year, Joe takes a week during the summer to relax at his friend’s cabin in the Maine woods. One night after he’s just arrived, he's sitting in the cabin when he hears a knock at the door. He opens the door and doesn't see anything -- until he looks down. On the wooden porch he sees a small snail. Annoyed, he picks up the snail and throws it as far as he can.
Three years later, Joe is back in the cabin for another summer retreat. There’s a knock on the door. He opens it and sees nothing, then remembers. He looks down -- and there’s the same snail!
The snail says: "What the hell was that all about?"