"A scorpion wants to cross a river but cannot swim, so it asks a frog to carry it across. The frog hesitates, afraid that the scorpion might sting it, but the scorpion promises not to, pointing out that they would both drown if the scorpion killed the frog in the middle of the river. The frog considers this argument sensible and agrees to transport the scorpion. The frog lets the scorpion climb on its back and begins to swim. Midway across the river, the scorpion stings the frog anyway, dooming them both. The dying frog asks the scorpion why it stung despite knowing the consequence, to which the scorpion replies: 'I couldn't help it. It's in my nature.'"

(http://www.autoadmit.com/thread.php?thread_id=4944324&forum_id=2,5#43288380)