The metaphor of children and lay adults as intuitive scientists has gained wide acceptance. Although useful in one sense, pertaining to scientific understanding, in another, pertaining to the process of scientific thinking, the metaphor may be fundamentally misleading. Research is reviewed indicating that processes of scientific thinking differ significantly in children, lay adults, and scientists. Hence, it is the instruments of scientific thinking, not just the products, that undergo "strong restructuring" (Carey, 1986). A framework for conceptualizing development of scientific thinking processes is proposed, centering on progressive differentiation and coordination of theory and evidence. This development is metacognitive, as well as strategic. It requires thinking about theories, rather than merely with them, and thinking about evidence, rather than merely being influenced by it, and, hence, reflects the attainment of control over the interaction of theories and evidence in one's own thinking.