Why plastic soda bottles have bumpy bottoms while water bottles don’t
The bumps are there to increase the strength of the bottle. This is necessary because the plastic bottle must be able to withstand the pressure of the gas in soda. Also, those drinks are usually served chilled, and when a liquid gets colder, its volume changes, and the bottle must not buckle because of it.As an extra, the corner or tip of the bump can slightly expand and contract without breaking. This also takes care of the volume change.