The aim of this research is to establish the contribution of demonstration and student-led experiments on the quality of students' knowledge about the air-related content in the third grade of primary school. This research included 120 students from Serbia, divided into two groups: control (C) and experimental (E). In group C the experiments were demonstrated by the teacher, whereas those same experiments were independently conducted by the students inside smaller groups in group E. The quality of the students' knowledge was examined after the application of the experiments with the post-test, while the quality of the duration of knowledge was examined with the retest. The students in the group E acquired the knowledge of the higher quality and more durable knowledge about the air-related content then the students from the group C on the cognitive levels: analysis, evaluation and synthesis. Therefore, the priority should be given to the student-led experiments over the demonstration, when processed air-related content in the third grade of primary school, because their application contributes to the increase of the students' knowledge quality.