"But Glass, in her research, discovered that if you dig a little deeper into people's infidelities, you can almost always see how the affair started long before the first stolen kiss. Most affairs begin, Glass wrote, when a husband or wife makes a new friend, and an apparently harmless intimacy is born. You don't sense the danger as it's happening, because what's wrong with friendship? Why can't we have friends of the opposite sex--or of the same sex, for that matter--even if we are married? The answer, as Dr. Glass explained, is that nothing is wrong with a married person launching a friendship outside of matrimony--so long as the "walls and windows" of the relationship remain in the correct places. It was Glass's theory that every healthy marriage is composed of walls and windows. The windows are the aspects of your relationship that are open to the world--that is, the necessary gaps through which you interact with family and friends; the walls are the barriers of trust behind which you guard the most intimate secrets of your marriage. What often happens, though, during so-called harmless friendships, is that you begin sharing intimacies with your new friend that belong hidden within your marriage. You reveal secrets about yourself--your deepest yearnings and frustrations--and it feels good to be so exposed. You throw open a window where there really ought to be a solid, weight-bearing wall, and soon you find yourself spilling your secret heart with this new person. Not wanting your spouse to feel jealous, you keep the details of your new friendship hidden. In so doing, you have now created a problem: You have just built a wall between you and your spouse where there really ought to be free circulation of air and light. The entire architecture of your matrimonial intimacy has therefore been rearranged. Every old wall is now a giant picture window; every old window is now boarded up like a crack house. You have just established the perfect blueprint for infidelity without even noticing."