Said when his mother suggested it's a good idea to put one's hand in front of one's mouth when coughing. I think he's been reading Nietzsche:
"He who fights with monsters should be careful lest he thereby become a monster. And if thou gaze long into an abyss, the abyss will also gaze into thee." - from Beyond Good and Evil, by Friedrich Nietzsche, at Chapter IV: "Apophthegms and Interludes," #146.