Introduction: A way to find out how scientific thinking in children develops is to focus on the processes that are involved. As such, scientific thinking can be seen as a particular form of problem solving in which the problem solver selects a strategy from the space of possible experiments that can reveal the cause of an event. Notwithstanding the acquisition of such a skill may be a strong constraint on search in the space of experiments, it was questioned whether children's problem solving could be supported by metacognitive instructions. Method: With the aid of a quasi-experimental study it was investigated whether children of 9 and 11 years old were able to design a scientific experiment with material available to them after they were taught how to control the variables in the experiment. A first group of 45 children was taught the technique together with metacognitive support (a step-by-step plan). A second group did so without metacognitive support. Finally, a third group received no specific instructions. Results. Results with 11-year-olds showed that children who were taught to design a scientific experiment together with metacognitive support outperformed those who didn't receive such instructions. In addition, children who received metacognitive support showed longer problem solving times indicating that their impulsiveness was inhibited. No significant differences were found with children of 9 years old. Discussion and Conclusion: Children should be encouraged to use metacognitive support in classrooms so that awareness on the level of their thoughts and actions leads to better scientific problem solving. (Contains 5 figures.)