When it comes to learning 'that' -- understanding the properties of our world -- another approach is needed.

In Bruner's terms, children need not only to know how, they need to know that. They need to know that liquids can be poured into different shapes but that the volume will remain constant, that written sentences end with a period, that objects can be grouped and regrouped into classes of objects, that parents cannot handle really loud sounds (unless they make them), that the color of liquids is a poor predictor of their taste or weight, that tone of voice is a key predictor of what a big sister will do next. They need to learn the properties of things, how they work, what the components are, what the relationships between properties are, and so on. How do children best learn these things?

Children are natural scientists. Piaget, Vygotsky, Bruner, and virtually every other developmentalist have noted that children are constantly searching to discover the properties of the world in which they live. Other than language, it is the curiosity of children, their unstoppable urge to explore and get into things, that most differentiates them from other species. Frogs and turtles are not curious; they are not scientists in the pond. Evolution has prepared them with perfectly adapted routines that are just right for the ecology of the pond.

Children do not come into the world so prepared. Much more helpless and dependent than most species, they must learn and construct what is special about their environment, what it's properties are. Evolution has prepared them not for a fixed environment like a pond but for many environments, each with different properties. Evolution has prepared them to learn what these properties are. Evolution has prepared them to play.