Consider two houses, one on the East Coast and the other on the West Coast, with
each house being home to a dozen kids. The kids in the East Coast household are
cousins of the kids in the West Coast household. The kids in the two households love
to write to each other—each kid writes each cousin every week, with each letter delivered by the traditional postal service in a separate envelope. Thus, each household
sends 144 letters to the other household every week. (These kids would save a lot of
money if they had e-mail!) In each of the households there is one kid—Ann in the
West Coast house and Bill in the East Coast house—responsible for mail collection
and mail distribution. Each week Ann visits all her brothers and sisters, collects the
mail, and gives the mail to a postal-service mail carrier, who makes daily visits to the
house. When letters arrive at the West Coast house, Ann also has the job of distributing the mail to her brothers and sisters. Bill has a similar job on the East Coast.