A doctor, a lawyer, and a physicist are having a drink, and the topic of conversation is: Is it better to have a wife, or a mistress? A wife, says the doctor: There are various diseases you are less likely to get. A mistress, says the lawyer: Then, if it doesn't work out, at least you're spared the agony of a divorce. A wife *and* a mistress, says the physicist: Then, when your wife thinks you're with your mistress, and your mistress thinks you're with your wife, you can do *physics*.