a scorpion, in the course of a long journey, encounters a river. the scorpion spies a toad at the opposite end of the river bank.

toad! the scorpion calls out, will you please let me ride on your back to the other side of the river because, you see, i cannot swim.

no way, replied the toad, if i let you ride on my back you will sting me!

to which the scorpion replied, but if i am riding on your back and I sting you then we shall both die 

after a short moment pondering the scorpion’s request, the toad agrees. halfway across the river, the scorpion stings him.

scorpion! exclaimed the toad, what have you done? now we both will surely drown

because, the scorpion responded, it is in my nature.