The Scorpion and the Frog

A scorpion, needing to get to the other side of the river, sees a frog on the bank. The 

scorpion asks, "Would you be so kind as to give me a ride on your back across the river?" 

The frog replies, "How do I know you won't sting me?" 

The scorpion says, "Because if I do you will drown and I will die too."

The frog is satisfied with the scorpion's reasoning, and they set out to cross the river. In midstream the scorpion stings the frog. The frog feels the pain and the onset of paralysis, and starts to sink. Knowing they both will drown, the frog gasps, "Why? Why did you do that?" 

Answers the scorpion: "It's my nature..."