Abstract: In this chapter is analysed the development of children's understanding of the concept of horizontal position of the liquid surface. Numerous studies have found that most children and approximately 40% of the adult population behave as if they do not know that liquid remains horizontal, regardless of the orientation of its container. The sample consisted of students from six classes in elementary school "Ivo Lola Ribar" and "Dositej Obradovic" in Sombor (Serbia). The project "Water is precious" was implemented in three of tham and lasted from March to June 2015. The paper presents the experience and difficulties in adopting corect concept about surface orientation of liquids. Based on the study findings, conclusions were drawn and possible solutions were offered. The study has once again confirmed that the one-time doing of experiments, without continuity in observation and experimentation is insufficient to properly and permanently adopt certain scientific notions and concepts such as horizontal position of the liquid surface.