The Frog and the Scorpion

A scorpion asks a frog to carry him across a stream. The frog asks:"How do I know you won't sting me?"
"If i do, both of us will drown," replies the scorpion.
The frog agrees. But as they are crossing the stream, the scorpion stings the frog.
"Why did you do that?" asks the frog as they both start to drown.
The scorpion just manages to answer:"Its my nature..."