Frog and the Scorpion

"One day, a scorpion stood on the side of a stream and asked a frog to carry it to the other side.
'How do I know you won't sting me?' the frog asked. 'Because if I sting you, I'll drown,' the scorpion said.
The frog thought about it and realized that the scorpion was right. So he put the scorpion on his back and started ferrying him. But midway across the stream, the scorpion plunged its stinger into the frog's back. As they both began to drown, the frog gasped,'Why?'
The scorpion replied, 'Because it is my nature.'