The Scorpion and the Frog

A scorpion and a frog meet on the bank of a stream and the
scorpion asks the frog to carry him across on its back. The
frog asks, "How do I know you won't sting me?" The scorpion
says, "Because if I do, I will die too."

  The frog is satisfied, and they set out, but in midstream,
the scorpion stings the frog. The frog feels the onset of
paralysis and starts to sink, knowing they both will drown,
but has just enough time to gasp "Why?"

Replies the scorpion: "Its my nature..."