A water bottle can be a useful system for the study of static equilibrium and stability. We have recently seen the fun physics of flipping water bottles explained by Dekker et al.1 The simple act of tipping them over is interesting as well. As a bottle is tipped, the liquid reshapes to find its level. If the center of mass of the system passes over the balance point, the bottle falls over. Otherwise, it is stable and returns to upright. Pahwa et al.2 determined the system center of mass geometrically and compared the theoretical tipping angle with experimental results. Licini and Yuan3 considered how a shampoo bottle becomes easier to knock over when it has less shampoo. We ask, How does the stability of a water bottle depend on the amount of liquid it holds, and with what amount is it the most stable? The problem is...

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