Most of what we know about children comes from watching carefully what they do. Proud parents entertain their friends - or bore them out of their minds - with the latest tales of their offspring's achievements. Just as frequently we discuss some 'problem', often asking a simple question such as, 'when did your child begin to walk?' or, 'when do they stop sucking their thumbs?' Teachers often ask more complex questions such as how children of a certain age can be expected to interact with each other, or how they learn difficult concepts. Checking with other experienced people helps. Reading textbooks helps. But watching children is both more interesting and more reliable.