A house catches on fire. The fire is extinguished by the combined efforts of (1) professional firefighters, (2) volunteer firefighters who help the professionals, and (3) passersby who spontaneously help the professionals and volunteers. The owner of the house makes various promises to induce the help of the three groups. Use economics to explain why a court should not enforce the promises, but the court should require the homeowner to pay (1) more than (2), and to pay (2) more than (3).