The Scopian

Once a scorpian sat upon the bank of a river which was too deep and swift for him to traverse. A frog swam and the scorpian called out, "Frog, carry me upon your back to the other shore!" The frog replied "I will not, for you will sting me and I will die."
The scorpian said to the frog "But why would I do that? For if i were to do that, I would drown."
The frog considered this argument, and at last said, "Very well. I shall carry you across the river."
So the frog came to the shore and took the scorpian upon his back and halfway across the river, the scorpian stung the frog.
With his last dying breath, the frog cried out, "Why have you done this? For now we shall both die!"
And with his dying breath, the scorpian said, "Because it is my nature."