Son Of A Magician

A school class are writing projects on their families. Little John is having trouble with his so the teacher helps him with a few questions.
"What's your father's occupation?" asks the teacher.
"He's a magician," says boy.
"How exciting. What's his best trick?"
"He saws people in half."
"Very impressive! And do you have any brothers or sisters?"
"Yea,” says John, “I’ve one half brother and one half sister."