To Spank or Not to Spank

I once heard this story and have concluded from it, that spanking is not always the effective form of discipline...

A man laid his son to bed one night and kissed him on the head. As he slowly walked out the door, his son yelled to him,
"Dad, can I have a glass of water?"
"No," the Dad said, "It's time to go to sleep."
The Dad walked out of the boy's room and headed down the hall.
"Dad, can I please have a glass of water?"
"No!" the dad said. "Go to sleep!"
"But dad, I really need a glass of water..."
Completely fed up and frustrated, the Dad yelled back, "If you don't go to sleep right now, I am going to come in there and give you a spanking!"
The dad heard a small pause of silence. "It worked," he thought. Until, his son yelled back,
"Hey Dad, on your way in to spank me, can you get me that glass of water?"