There's a rule of thumb for just about every subject imaginable - money, marriage, cooking, health, weather, cars, gardening, restaurants, travel. This is the priceless knowledge that's accumulated not from reading, but from living. Tempted by a string of pearls at a yard sale? Rub them against your teeth: the rule of thumb, as your grandmother might have told you, is that a genuine pearl will grate. Not sure how to choose a puppy? Pick one whose tail wags in sync with its walk - a sign of calmness. To win at blackjack, assume that any unseen card is an eight. And remember, a rule of thumb works four out of five times (including this one).