The theory of cause and effect is fundamental to our understanding of the world. However, despite almost universal agreement that children learn about cause and effect through exploratory play, little is known about how children's play might support accurate causal learning, Schulz said.

"One of the deep mysteries of cognitive science is how we predict the future and how we explain the past and intervene in the present," she said. Causal reasoning even pervades our emotional lives when we speculate about why someone has a certain expression on her face or why a friend or colleague said what he did.

Statistical evidence is one factor that contributes to our rich beliefs about the universe. Our prior experiences and beliefs affect how we interpret the evidence we see with our own eyes or hear about from scientists. But how do children form their conclusions about how the world works?

Schulz is continually amazed at how much knowledge of how things work--not only mechanically, but also socially and culturally--children pick up in early childhood. "Everything you need to know about the world you really did know before kindergarten," she said.

Schulz does not doubt that children pick up much of their knowledge about how the world works through play. But child's play is messy--not just because of paint and juice, but because it can look random and unfocused to an observer. The trick is to design experiments that isolate the variables, she said.