Developing critical thinking skills in future citizens is a major educational challenge for twenty-first century schools. Defining critical thinking is complex, leading to diverse educational approaches. One promising, though less common, approach is transversal—focusing on the cognitive processes that underlie critical thinking. This approach has mostly been studied with older students, leaving a significant gap in the research on its effectiveness with primary school learners. This study addresses that gap by evaluating a targeted intervention with 92 fifth-grade students aimed at helping them recognise and counteract three common cognitive biases. The 12 weeks intervention consisted of an explicit teaching sequence designed to raise students’ awareness of the correlation-causation bias, the confirmation bias and the argument from authority fallacy. Sessions on the latter also addressed source checking. We performed 16 tests, and the results showed significant improvements in 14 of them. Students better identified correlation and causation ( p < .01 in all setups), countered confirmation bias ( p < .01 in two setups), and identified trustworthy sources for decision-making ( p < .01 in all setups). They also chose multiple sources rather than relying on a single one ( p < .01 in all setups). These findings suggest that brief and targeted interventions can improve critical thinking skills in young learners, encouraging further exploration of domain-general approaches to develop elementary school students' critical thinking skills and help them become enlightened, responsible citizens.