The Doctor's grin fades a little in his confusion. "Your mum?"

"How long has she been your mum -- Weell," he reaches out and puts his hand on Luke's head then moves it to his own chest, measuring their heights against one another, looking him up and down. "Thirteen, fourteen years maybe? When are you from?"

At least if she's told Luke about him, that question won't be quite as baffling.