You're about to leave work at the end of the day when your cell phone rings: it's your spouse, asking that you pick up a gallon of milk on the way home. Before you head out the door, though, your spouse calls again and asks you to stop by the hardware store too. Based on your knowledge of the area and rush-hour traffic, you decide to get the milk first and the toilet plunger second. But whoops! The phone rings again. This time, it's your boss, asking you to work late. That means another change of plans.