The study provides evidence concerning elementary school children's ability to conduct a scientific investigation. Two hundred and fifty sixth-grade students and 248 fourth-grade students were administered a test, and based on their performance, they were classified into high-ability and low-ability students. The sample of this study was randomly selected and included 80 students, 40 fourth-grade and 40 sixth-grade students of low and high abilities. Students were specifically instructed to investigate the functioning of a device, to think aloud prior and after any experiment with the device, and to keep a record of their experimental results. The results showed that students were inclined to mainly collect evidence from the experimental space and failed to control variables during their investigation. The majority of the students had difficulties with effectively organizing collected data and failed to coordinate hypotheses with evidence. The significant interaction effect that was found between grade level and ability in terms of students' investigation ability indicates that the existing gap between high- and low-ability students becomes bigger as students become older. Undoubtedly, ongoing research efforts for identifying patterns of children's cognitive development will be most valuable as they can have important implications for the design of teaching scenarios and inquiry-based science activities conducive to accelerating students' cognitive growth and scientific investigation abilities.