[ponygirl]

It was always my understanding that the popout on the side of the milk carton was to warn that the milk had gone bad and expanded.

[TangoLimaSierra]

It was always my understanding that the popout on the side of the milk carton was to warn that the milk had gone bad and expanded.

That also. Here's the 3 reasons for the dimple:

This Is Why Milk Jugs Have Those Circular Dimples on Their Sides