In our study we probed students’ account of the profile of water in a vessel placed in a variety of physical settings. We found that this knowledge can be interpreted in terms of schemes-of-knowledge – elementary conceptions of a general nature that are used by learners to account for physical situation. The six elicited schemes (all with a clear historical analogue in the history of science) widely employ concepts resembling the physical account within the non-inertial frames of reference, currently excluded from the standard curricula, but being in a wide use in the everyday life and in engineering. The students often reason by inertial forces also while arriving at the correct answers. We think that teachers should not blindly suppress this reasoning as incorrect but carefully elaborate it using this opportunity to introduce the important idea of observer dependence of physics description. This approach might suggest a significant change in the training program of physics teachers.