The Combinations Rule: underlines creative thought
Stan's right: this is the number one rule for any creative venture, including some of the best teaching that I ever received (and that I've seen practiced through the profession's lens). The best teachers always seemed to know a lot about stuff that had nothing to do with the 'subject' in hand. They always knew the names of plants, birds, trees, obscure books and films.
It reminds me of something I read, maybe in The Art of Looking Sideways, that to be interesting you have to be interested. In everything.