What is a wall? Keynoting their venture with a Robert Frost quote (``Before I built a wall I'd ask to know/What I was walling in or walling out...''), Knight and O'Brien explore the concept in 14 two-page presentations of notable walls from six continents and many periods: defensive (China's Great Wall, Berlin, the walls of Mandela's prison); walls holding prehistoric artwork in Australia and France, as well as Rivera's frescoes; historic sites (Jerusalem's Western Wall); the Vietnam Memorial. The brief, straightforward text has additional concluding notes; the large, double-spread illustrations are in a relaxed, impressionistic style, expressive and lively with activity— including a multicultural cast of modern children at the sites. Thoughtfully composed, an unusually attractive book that wisely allows readers to draw their own inferences. (Nonfiction/Picture book. 4-10)