The classic wood unit block set was designed in the early 1900s by Caroline Pratt, founder of the City & Country School in Manhattan. They have been an important part of early childhood classrooms ever since. Built to scale, the blocks allow children to experiment fitting parts to a whole: there is the central unit block, matched by the double unit and quad. As preschool director, I no longer have my own classroom. But when I did, the block area occupied nearly half of the space in my room. Because of this, and because we highlighted the process in so many ways, those 4-year-olds were rockin’ block builders.