the scorpion + the frog

One day a scorpion was walking along the riverbank trying to find a 
way to get across the river. He saw a frog sitting alongside the riverbank.
The scorpion walked up to the frog and asked him if he would take
him across the river. 

The frog said, "No, I will not give you a ride." 
When the scorpion asked why, the frog replied,
"Because if I give you a ride on my back you will sting
me and I will die." 

The scorpion promised not to sting him, but the frog said, "No, I don't trust you."

The scorpion replied, "Perhaps you don't trust me, but you should trust logic. 
Mr. Frog, if I stung you, then you would drown. If you drown, then I would drown too,
since I cannot swim. There is no logic in me stinging you." 

The frog thought for a moment and then said, "I guess you're
right. If you promise not to sting me, I will give you a ride." 
The scorpion made his promise and jumped on the frog's back,
so they could start across the river. 

Halfway across the river, the frog suddenly feels a sharp pain in his back, 
as the scorpion stings him.
 The frog immediately starts to panic as he feels the venom race
through his veins and begins to become paralyzed. Just as
he is taking his last breath, the frog looks at
the scorpion and asks "Why did you do it? You promised not to sting
me! Now we are both going to die!" 

The scorpion replies, "I'm sorry, Sir, but I could not help it - it's my nature".