There's a post I saw on reddit that points to the dimple on the side of a milk jug, and makes fun of all the people who don't know what that's for. In the comments are thousands of people giving dozens of different explanations, and all of them are wrong.
It is not there to indicate that the milk has spoiled by popping out due to gasses produced by spoiled milk. If there was enough gas to pop out the dimple, the whole jug would look like a balloon.
It is not there to provide structural integrity, like lateral support to prevent the bottles from crushing. The contents are under pressure, so if there was enough force on the jug from any direction, then the cap would pop off regardless of the shape in the sidewall.
The actual answer is that the dimple is added to ensure that all of the jugs contain the same volume of milk. Plastic jugs are blown into molds, and minor manufacturing variations over time would create jugs that hold different amounts of milk. Larger jugs would hold more than a gallon. They could just fill by volume, but consumers are wary of purchasing a bottle if it appears to be less full than the others. So they add the dimple to make it so that the level of milk is all the way at the top with minimal air between the milk and the cap.
You can verify this yourself by finding different jugs from the same supplier with dimples of different depths, or even no dimple at all. None of those other explanations would explain dimples of different sizes or jugs without dimples.
TLDR everybody is wrong. The milk jug dimples are added to ensure the jug contains the correct volume of milk.
You're welcome and encouraged to look into it yourself. You misunderstand what I'm saying and draw further conclusions based on that, though, so I can see why it doesn't make sense. I'll take a stab at explaining.
I did mean surface area, not thickness. As volume decreases, so do the dimensions of the object. The thickness of the plastic is already negligible and any change within that plane is a fraction of that, so even less pertinent here. The remaining two planes of the exterior, being several orders of magnitude larger, do experience functionally significant, easily measured change. Those two planes as they relate to volume are most succinctly explained as surface area.
I mentioned the SA:V change to illustrate that this size change isn't visually apparent, so it's important to adjust the volume via the dimple. This maintains a steady milk level so jugs can hold an entire gallon in the winter and ensures customers don't think jugs are underfilled in the summer. In cold weather, the dimensions of the jugs reduce less than 1%, which means visually the change is difficult to notice, but the volume changes a fair amount, around 5%. A change in size imperceptible to most reduces the volume of the jug by about 1/20 without compensation*. By reducing the size of the dimple, less plastic can be used, which saves money.
So I'm simpler terms I think what you're saying is the bottle shrinks (surface area decreases). When it does, the volume decreases too and it's a 5:1 ratio. Correct?
I did click on the link someone posted here and the article made sense to me. Number one was structural. Which is what I would think. Number two mentioned this winter summer think but not during manufacturing ( which makes sense to me), but during shipping which still doesn't make sense to me. What milk or even orange juice is NOT refrigerated?
Bingo, you got it. It only shrinks so much in volume because there's a ton of surface area. If it was something with less surface area in relation to volume, like a sphere, it'd be much less dramatic.
Yeah, the shipping thing doesn't really make sense for exactly the reasons you stated. The issue is more about the temperature when the bottle is made. This is done by inflating a warm plastic tube in a mold. Cooler plastic has a bit more rebound, where it shrinks a little when the air pressure is removed. This still happens in the summer, but since the environment is a touch warmer, it shrinks just a bit less. Since a 0.5-1.0% overall change has a significant effect on the volume, you get the compensating dimple.