"What's your book about?" Definitely the hardest, and I usually don't even answer it, except to give the genre.
Where do you get your ideas is easy. I just tell them I get all my ideas from Pro Writer's Idea Magazine. I explain that once you have two novel sales to large publishers, or twelve short story sales to national magazines, you're eligible to receive a subscription, which, at $400 per year, is dreadfully expensive, but well worth the cost because, each month, it contains more than a hundred ideas, along with a mini-outline of how the story might best be written. The ideas and mini-outlines are all written by bestselling writers, and having a subscription to Pro Writer's Idea Magazine gives any writer a HUGE advantage over new writers.
This, of course, is one reason many new writers have trouble breaking in, but we had to earn our way to a subscription, and so should everyone else.
It's amazing how mad many new writers get up hearing about Pro Writer's Idea Magazine, and their ineligibility.