Johnny is playing fetch with his dog in the park, when he accidentally throws the stick onto a lake
To his amazement, the dog runs onto the lake, walks across the water, and brings back the stick.
Johnny can’t believe his eyes, so he throws the stick onto the lake again, and once more the dog walks on the water and fetches the stick.
A man comes walking by, and wanting to show off his dog’s ability, Johnny calls to him, “Hey, watch this!”. Then he throws the stick into the lake and once again, the dog walks on the water to fetch the stick.
Grinning with pride, Johnny turns to the man and asks “Well, did you notice anything unusual about my dog?”
“Yes” the man replies, “Your dog can’t swim”