Maths homework

A little boy is doing his math homework and while he’s working out his sums he says to himself, “Two plus five, that son of a bitch is seven. Three plus six, that son of a bitch is nine...”
His mother hears what he’s saying and gasps, “What are you doing?”
“I’m doing my maths homework.” The boy replies.
“And is this how your teacher taught you to do it?” the mother asks.
“Yes,” he answers.
Infuriated, the next day the mother asks the teacher, “What are you teaching my son in math?”
“He’s learning addition.” The teacher replies.
“And are you teaching him to say two plus two, that son of a bitch is four?” Ask the mother?
After the teacher stops laughing, she answers, “No. No. What I taught him was, two plus two, THE SUM OF WHICH, is four.”