I lend lots of people lots of books.

I always know how smart someone is when I lend them a book.

If they come back and either dismiss the book out of hand, give a reason why they don’t like the writer for some random personal reason, or sum it up in a quick synopsis as if it’s a 1 minute conversation, I realize instantly that they’re not smart and (my lending it to them was a waste of time).

If they make a screwed-up face and then struggle for the next 3 hours to explain what they have read, hopefully with passionate back-and-forth discussion, then I know I was right to lend them the book.