Constructive play begins with the kind of large corrugated cardboard blocks beloved by generations of children. They are light enough even for toddlers to carry and stack, big enough to show quick results, and strong enough to stand or sit on. Their proportional sizes, based on a 6"x6" square that is 3" high, also include half-squares and double squares; you can easily see how preschoolers will learn the concepts of addition, equality, and fractions even before they can understand numbers.