Peter Williss, age 3 ?, learns difference in weight with the aid of three boxes containing pieces of wood. The pieces in the first box are 5 grammes heavier than those in the second and those in the third box being ton grammes heavier than those in the first box. Heaviest blocks are dark coloured, medium weights are lighter and the lightest in weight are lightest in colour. Blindfold Peter sorts them into piles and then lifts the cloth to check his results. At Netherton Nursery School, London. December 20, 1949. (Photo by Fox Photos).