Researchers, educators, and parents have long believed that children learn cause and effect relationships through exploratory play. However, previous research suggests that children are poor at designing informative experiments; children fail to control relevant variables and tend to alter multiple variables simultaneously. Thus, little is known about how children's spontaneous exploration might support accurate causal inferences. Here we suggest that children's exploratory play is affected by the quality of the evidence they observe. Using a novel free-play paradigm, we show that preschoolers (mean age: 57 months) distinguish confounded and unconfounded evidence, preferentially explore causally confounded (but not matched unconfounded) toys rather than novel toys, and spontaneously disambiguate confounded variables in the course of free play.KEWORDS: causal learning, exploratory play, preschoolers' scientific reasoning, ambiguous evidence, confounded variables CONFOUNDING AND PLAY 3 Serious fun: Preschoolers engage in more exploratory play when evidence is confounded Causal knowledge is fundamental to our understanding of the world. It informs our moral judgments, our explanations of the past, and our plans for the future. Little wonder that Hume called causal knowledge the "cement of the universe" (1740/2000). However, relatively little is known about how children learn the causal structure of events.Piaget believed that young children came to understand causal relationships through active exploration of their environment (1930). Recent research suggests that very young children know much more about the causal structure of the world than Piaget believed (Baillargeon, Kotovsky, & Needham, 1995;Flavell, Green, & Flavell, 1995;Gelman & Wellman, 1991;Kalish, 1996;Leslie & Keeble, 1987;Saxe, Tenenbaum, & Carey, 2005;Spelke, Breinlinger, Macomber, & Jacobson, 1992). However, the idea at the heart of the Piagetian account --that children "construct" knowledge (and particularly causal knowledge) by active exploration --remains widely accepted.Yet despite substantial agreement that children learn through play (e.g., Bruner, Jolly, & Sylva, 1976;Singer, Golinkoff, & Hirsh-Pasek, 2006), little is known about how exploratory play might support accurate causal inferences. Descriptive studies of play are more common than experimental research or theoretical accounts, and much of the seminal work on exploratory play predates recent research on children's causal reasoning (Berlyne, 1954;Piaget, 1951). Except for the well-established finding that children (and many non-human animals) selectively explore novel stimuli (Berlyne, 1960;Dember & Earl, 1957;Henderson & Moore, 1980; Hutt & Bhaynani, 1972;Pavlov, 1927), there is CONFOUNDING AND PLAY 4 little evidence for systematic patterns in children's exploratory behavior. Moreover, considerable research suggests that even older children and naïve adults are poor at designing causally informative experiments and have difficulty anticipating the type of evidence that would s...