The aim of this paper is to investigate the stability of a bottle filled with different volumes of water, and to determine the angle at which it topples over for each volume of water. Data for the angle at which the bottle toppled were gathered experimentally using a cylindrical 1 litre bottle, and two theoretical models were then developed, considering the fluid as both static and dynamic, respectively. By modeling the water as a uniform cylinder or as a rectangular cylinder and cylindrical segment, it was possible to calculate the angle at which the centre of mass of the system overshot the base of the bottle. Allowing for some discrepancies between the geometry of the actual bottle and that of the idealised model, it was found that both models could successfully predict the variation of the toppling angle with volume. However, it was found that the second model, which treated the fluid dynamically, was more appropriate, and better supported by the empirical data gathered. These data and the models also indicated that the toppling angle was found to be highest when the bottle was approximately one-third full with water. Using an Excel sheet for theoretical calculations it is possible to predict the angle at which cylindrical bottles of various sizes filled with fluid can topple over.