A drink of water

A boy is sent to bed by his father.
Five minutes later, he calls down. "Daaad...."
"What?" the father says.
"I'm thirsty," the boy says. "Can you bring me a drink of water?"
"No," the father replies. You had your water already. Lights out."
Five minutes later, the boy once again calls downstairs. "Daaaaaad....."
"WHAT?" the father snaps.
"I really want a drink of water. Can I please have one?" the boy asks.
"I told you no!" the father asserts. "If you ask again, I'm coming up there to spank you!"
Five minutes later, the boy calls downstairs again, "Daaaaaaaaad....."
"WHAT?" the father once again snaps.
The boy replies, "When you come up here to spank me, can you bring me a drink of water?"