The Turtle and The Scorpion:
A turtle was happily swimming along a river when a scorpion hailed it from the shore.
The scorpion, being a very poor swimmer, asked the turtle to carry him on his back across the river.
"Are you mad?" exclaimed the turtle. "You'll sting me while I'm swimming and I'll drown."
"My dear turtle," laughed the scorpion, "if I were to sting you, you would drown and I would go down with you, and drown as well. Now where is the logic in that?"
The turtle thought this over, and saw the logic of the scorpion's statement. "You're right!" cried the turtle. "Hop on!"
The scorpion climbed aboard and halfway across the river the scorpion gave the turtle a mighty sting. As they both sank to the bottom, the turtle resignedly said:
"Do you mind if I ask you something? You said there'd be no logic in your stinging me. Why did you do it?"
"It has nothing to do with logic," the drowning scorpion sadly replied. "It's just my nature."
-- Fable; Origin and author unknown; Attributed to Nur ad-Din Abd ar-Rahman Jami