Frog and the scorpion
A scorpion asks a frog to carry it across the river. The frog hesitates, afraid of being stung.
“But” the scorpion argues, “if I stung you, we would both drown”.
“Right”, said the frog, and agrees to carry the scorpion.
They wade into the river. Midway across, the scorpion stings the frog.
The frog says, “What the hell did you do that for? Now we’re both screwed!”
The scorpion shrugs. “It's in my nature,” he says.