It's not so much the ignorant questions (there's no way for them to learn unless they ask) that get me but the often silly, dismissive remarks made after the questions get answered. Like, "That's dumb!" or "I wouldn't waste time on real plants - I would just get fake ones."
My favorite are remarks about the expense involved in the hobby. Usually from people who are in suits that cost more than MacBook Pro or from folks wearing $25K watches or earrings. I just smile and nod.