The present work refers to a study of 10-12-year-old students' ideas on the self- or hetero-luminosity of the Moon. To check the validity of students' reasoning on this question, we also examined their ideas on the lunar phases and how we see hetero-luminous objects. Students supported their ideas on the self- or hetero-luminosity of the Moon by using naïve explanatory reasoning, based on alternative ideas about various astronomical phenomena. On the other hand, an analysis of the students' answers showed that there are qualitative differences between the two groups. For students to understand that the Moon is illuminated by the Sun, comprehension of the relative movements, relative sizes and distances of the Sun-Earth-Moon system is necessary.