A Snake in the Grass

A young man was walking along in the woods one chilly spring day when he suddenly came upon a snake laying motionless in the middle of the dirt path.  He gently prodded it with his hiking staff to see if he could coax it to slither aside.  "Please, young sir", implored the snake, "I am but a cold blooded animal and I cannot gather the strength on this cool day to crawl back to my nest to guard my young.  If you would but pick me up and warm me for a few minutes I could summon the energy to wriggle home and check my brood".
The young man was taken aback by the snakes odd request.  "What kind of fool do you take me for?" he asked the snake.  "You would most assuredly bite me".
"Oh, no.  I am way to weak to even think such a thing.  I am only interested in my young.  Please, please.  I beg of you.  My children may themselves be killed by a hungry mammal if I am not there to protect them".
"I don't know", replied the man, obviously beginning to soften to the snakes pitiful circumstance.  "Do you promise not to bite me if I help you?"
"Of course, of course.  I would never bite the hand that helped me".

So, perhaps against the young man's gut sense, he knelt down and extended his hand for the snake to crawl onto his arm to be warmed.  And just as he opened his palm, the snake struck out and sank his fangs deep into the soft flesh of his hand.  "Ouch!" he cried out in pain and astonished disbelief.  " Why did you do that?  You gave me your word".
"You knew what I was when you reached out to hold me", hissed the snake as he slithered away.  "You should not have expected me to be something I am not".