system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are the bottom of plastic bottles a funny shape and why can't it just have a flat bottom (eg. Mountain Dew, Pepsi ect)?

My best guess would be to do with balance? But not sure how it helps?
gpt
I am old enough to remember when plastic soda bottles did have flat bottoms -- it was a separate piece, like a cap. The part containing the liquid was round, like the bottom of a test tube, and that sat inside a glued-on black plastic piece.  Without that piece, the bottle would not be able to stand up.  
Then someone figured out how to make the little feet-shaped things and bottles became just one piece of plastic, easier to manufacture as others have mentioned, and avoiding the bulging-bottom situation.