After 20 minutes of waiting outside the baggage claim area for your friend to pick you up, you decide to go inside the airport terminal and phone her. Standing a short distance from you is a kind-looking older woman. Would you mind watching my luggage so I can go make a quick phone call?" you ask politely. I'd be happy to watch it for you," she replies, "My ride is not coming for another half-hour." When you return less than five minutes later, the old lady and your luggage (including your laptop) are gone. This example illustrates one of the problems associated with: