A shepherd is tending his sheep, and a man comes by and says, "If I guess the correct number of your sheep, can I have one?"
The shepherd says, "Sure - please try."
The man looks at the flock and says "Eighty-three."
The shepherd is completely amazed that he got the right number. The man picks up a sheep and starts to walk away.
The shepherd says, "Wait! If I guess your profession, can I have my sheep back?" The man says, "Sure."
The shepherd says, "You must be a mathematical biologist."
The man says, "How did you know?"
"Because you picked up my dog."
(via the always excellent Carl Zimmer)