A dynamic assessment tool was developed and validated using Mokken scale analysis to assess the extent to which kindergartners are able to construct unconfounded experiments, an essential part of scientific reasoning. Scientific reasoning is one of the learning processes happening within science education. A commonly used, hands-on, experimentation task was adapted to dynamically assess the use of the so-called control of variables strategy (CVS) by children 4 to 6 years of age. In this task, the children were challenged to design experiments using two ramps with up to four independent variables: weight of the ball, steepness of the slope, place of the starting gate, and surface texture of the slope. There were two scores of CVS use: experiment and variable correct score. The analysis showed it was possible to assess CVS use in a reliable and valid manner with the new assessment tool. Irrespective of the number of variables children were allowed to set, experiments validly measured CVS use. Given that the number of variables to be set increased the difficulty of the experiment, this can be used to scale children's CVS use. In other words, it is possible to differentiate between children on the basis of their CVS use. The children's use of CVS positively related to both their age and nonverbal reasoning ability. The present results thus show that it is feasible to evaluate the ability of kindergartners to construct unconfounded experiments using dynamic assessment. This means that kindergartners can use the CVS and might be seen as natural scientists, who can and will try to unravel the physical world around them. Their explorations appear to need sufficient guidance (i.e. within their zone of proximal development) to design multivariable experiments.

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Inquiry-based lessons have been demonstrated to improve children's scientific thinking (i.e. reasoning abilities and domainspecific knowledge). Although empirical evidence shows that inquiry-based learning requires instruction, research comes from two approaches that have not been bridged yet: direct instruction of scientific reasoning and teacher training of verbal support. We investigated how these two types of instruction separately or combined strengthened children's scientific thinking by comparing four conditions: baseline, direct instruction, verbal support, and a combined approach. Effectiveness of an inquirybased lesson series on scientific reasoning abilities, vocabulary, and domain-specific knowledge (near and far transfer) were studied among 301 fourth graders. Results showed that both approaches strengthened different components of scientific reasoning abilities, and that a combination of instructions was most effective for scientific reasoning abilities, vocabulary, and domain-specific knowledge. Domain-specific knowledge acquisition was strengthened only when both instructions were provided. It can thus be concluded that each type of instruction has unique contributions to children's science learning and that these instructions complement each other. Our study thus showed that inquiry-based lesson series when preceded by direct instruction of scientific reasoning and scaffolded with verbal support are most effective. ARTICLE HISTORY