Quantum physics is challenging for young students, but also a source of fascination. "Observation" is a key concept in order to understand how principles and experimental results in quantum physics differ from what we are used to in classical physics and everyday experiences. In this study we investigate how pre-university physics students understand the concept of observation in the case of the famous double-slit experiment with electrons and interpretations of its results. We found that a conception of observation as "looking," meaning a passive registration, is prevalent among students. This causes serious problems in understanding quantum physics and leads to very unproductive speculations that links to mysticism. Some students considered observation as measurement involving some sort of apparatus, but very few expressed the key idea of measurement as interaction. We discuss how a more explicit discussion of what observation means in quantum physics can benefit students' understanding of principles in quantum physics and their philosophical consequences.