The purpose of this research was to explore concepts about thinking that contribute to children's metacognitive experience during experimentation. Twenty-four randomly selected elementary school children in Toronto, Ontario, Canada, participated in five theory of mind tasks that assessed their: understanding of instantiation inference; understanding of causal and non-causal inference; understanding of causal and non-causal evidence; understanding of critical and non-critical evidence in a hypothesis testing format; and understanding of critical and non-critical evidence in a referential communication task. It was concluded that: children progress from unplanned to planned experimentation; the relationship between plans and procedures changes with grade level, so that early procedures do not depend on plans, but later procedures do; planning contrastive experiments does not depend on understanding causal inference; children's ability to direct and explain their own reasoning in science experimentation tasks was at the same level as their ability to affect and explain the beliefs of others in theory of mind tasks; and theory of mind appears to operate in two aspects of experimentation, in the productive phases it guides procedures by supporting the child's planning while in the critical phases it facilitates acceptance of disconfirming evidence and allows children to explain how their observations justify a particular inference. (JRH)