George is playing fetch with his dog by a lake.
He throws a stick out on the water and the dog trots out atop the water, retrieves the stick and walks back to him. George is flabbergasted. He throws the stick again. And again, the dog trots out atop the water, gets the stick and brings it back.
The man can hardly believe his eyes! Bursting with excitement, that evening he goes to his neighbor's house, Bill, and invites him to come down to the lake the next day, hoping to show off his amazing dog.
Once they arrive, the man throws the stick out into the middle of the lake. Just as before, the dog trots out on top of the water, grabs the stick, and trots back. Bill watches calmly and says nothing.
George throws the stick again. The dog walks on the water, gets it, and returns. Still, the neighbor remains silent.
Unable to contain himself any longer, George asks, "So... did you notice anything unusual about my dog?"
Bill rubs his chin and replies, "Yeah, I noticed. He can't swim, can he?"