Mulla walked into a clothing store, picked out a pair of pants, tried them on, and started walking toward the door. After a few steps, he changed his mind, walked back, took off the pants, and said to the clerk, "Actually, there's nothing wrong with these pants, but I'll trade them for a vest instead." The clerk handed him a vest. Mulla put it on and started walking out. "Hey!" the clerk yelled. "You didn't pay for the vest!" Mulla looked confused. "I gave you the pants for the vest." "But you didn't pay for the pants either!" Mulla scoffed, "Why would I pay for pants I didn't even take?"