Experimental science activities in primary school enable important cross-curricular learning. In this study, experimental activities on microbiology were carried out by 16 pupils in a Portuguese grade-4 classroom (9-10?years old) and were focused on two problem-questions related to microbiology and health: (1) do your teeth carry microbes? (2) why should you brush your teeth after meals? To solve problem-question (1), children's samples of dental plaque were prepared and observed under the microscope. For question (2), culture medium plates were inoculated with children's dental plaque either before or after tooth brushing and the colonies were counted. Results showed that pupils easily recognised the presence of microorganisms in the mouth and verified the effectiveness of the process of teeth-brushing. Pupils understood that microorganisms were potentially responsible for the onset of dental caries. These practical activities on microbiology in primary school were very effective in ensuring young children easily understand the causes of dental caries and how they can be prevented. Pupils became aware that the act of brushing their teeth is not only a socially correct behaviour or a simple rule to meet, but also a matter of preventing illness and promoting health.