What killed the frog?

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 it would drown if it killed the frog in the middle of the river. The frog considers this argument sensible and agrees to transport the scorpion. 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 am sorry, but I couldn't resist the urge. It's in my nature."