New friend Bernie suggests Margot's mother never leaves their fourth floor walk-up because she is ""just about the fattest woman I have ever seen,"" but it's not that simple, which is why eleven-year-old Margot has such a hard time accomplishing her Summer Homework project: to get mom downstairs. After nine years at home, Mrs. Green maintains ""I'm happy where I am,"" and her defense seems impregnable; even dad, a touring flutist, no longer presses her, and to her credit, she has made upstairs an attractive, independent enclave. In another context, versatile Bernie mentions Newton's Law--he also introduces her to Marx Brothers movies and New York's finest haunts--and Margot sees in the scientific principle a good omen. But when her well-meaning scheme misfires she takes drastic action, forcing Mrs. Green and the issue out into the open--a momentary hollow victory. Father, returning soon after, wisely counsels, ""Life isn't like a magic show with amazing transformations,"" and Margot, wised up, agrees. Hurwitz, a children's librarian, skillfully integrates New York City landmarks into the story (including Claudia's bed from Mixed-up Files) and handles the mother's intractability with uncommon care, a sensitivity echoed in Fetz' scratchy, aptly placed drawings. Bernie roundly earns his place in that long line of irresistibly resourceful children, Margot has her own resilient charms, and the smoothly concoted urban scene goes down as easily as Duck Soup.