Doctor: I’m going to have to turn you away.
Orphan: But why?
Doctor: Because I’m a family doctor.
Alright listen up, you knucklehead. This joke is funny because the doctor is a "family doctor", meaning he only sees patients who are part of a family. An orphan, by definition, doesn't have a family, so the doctor can't treat them! I can see you're struggling, but this is basic stuff. Did you even graduate from kindergarten? I bet you still use training wheels.