One fall day, Mom Margie and her two children--Dennis, nine, and Susie, eight--happily walk on a two-way bicycle path surrounded by trees that is frequented by walkers and joggers as well as bicyclists. They listen to birds and watch squirrels run across the path, up the trees, and chase one another. Occasionally, someone walks by, pushing a carriage and/or walking a dog. Dennis turns to his mother. "Can we get a pet? We really want...