This paper is essentially about the general form of Newton's second law for variable mass problems. We develop a model for describing the motion of the one-dimensional oscillator with a variable mass within the framework of classroom physics. We present a simple numerical procedure for the solution of the equation of motion of the system to be implemented by students and teachers. Interesting qualitative concepts as well as quantitative results for the focused problem are presented. The topic has pedagogical value from both the theoretical and experimental point of view. However, this paper considers only theoretical aspects of the problem. It is aimed at basic physics courses at undergraduate level.