Hello, Mr Gordon. Why don't you come to the lab and I'll explain why does that happen when you open your milk.
Hello Dr.Boyle. This morning I was opening the jar with milk and for my surprise when I removed the lid it jumped and made a sound, PLOP!
Oxford, Great Britain 1662
You see this jar of milk, when it was sealed from the farm, there was air left inside the bottle, and the particles in this gas are moving really fast which is the pressure, and the space they have to move is the volume. When the lid is removed, this gas particles escape moving really fast as they are in need for more volume. The relation between the pressure and the volume affects the temperature of the gas. I call this Boyle's Law.
The formula to understand this behavior is P x V = Constant