ssure valves are often round.
A Small Detail With a Big Purpose
The circular dent on a milk jug is a perfect example of invisible design—a feature that works so well you never notice it. It protects against spills, saves material, adapts to pressure changes, and even offers clues about spoilage, all without adding complexity.
So the next time you pour a glass of milk, take a second look at that little circle on the side of the jug. It’s quietly doing a lot more work than most people will ever realize.