Why are milk cartons square or squarish, but soda bottles round?

Have you ever wondered why milk cartons are square but containers for soda bottles are round? I have. I was talking to a former engineer that I know recently and he suggested a simple explanation. First, one would think that there is a benefit to make all the containers square because less space will be wasted in stacking the containers. Rounded containers end up having a lot of air space between them, and the extra air space also would make it harder to keep the milk cold. So why aren't soda containers square? In a single word: pressure. Round containers deal much better with pressure than square ones. Even if you put the soda in a square container, the pressure would push out the sides so that the container would become more rounded in any case.