A Kid Comes Home After School

A kid comes home after school and shows his father his grade book. The father looks at it and looks back at the kid and angrily says "You got an F today? Well, you are grounded. And if you don't get a good grade tomorrow you are not my son anymore!"

The next day the kid comes home again. The father says "What grade did you get today?" and the kid replies "Excuse me, who the fuck are you?"