Frog and the scorpion

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