How I Make a Living

So yesterday I have these patients come in. (All names have been changed to protect the stupid). Grandma, mom (who's 17) and little Billy, who's 2. The complaint is that Billy's bum hurts. He's been complaining about it intermittently for a couple of weeks. So I ask: what's the concern today? And mom, bouncing Billy on her lap looks at him and says "Tell him: 'my bum hurts!'"
So I look at mom and say "Wait, it's your bum, or his?" with a joking smile on my face. And to her credit, mom gets it and goes "No, his bum!"
And then grandma looks over and says to her daughter: "Wait, your bum hurts too?"