Hello, I'm new here. I hope I do not offend all of you physics gurus with my simple question. Here is the scenario: I have a Rubbermaid 2.5 liter container that I like to fill up with hot (boiling) water, 3 teabags and few spoons of sugar. I'm not sure what kind of plastic exactly it is, but it does not melt or shrink like a regular PET bottle. It is pretty sturdy. When I close the lid and shake the hot tea inside to make it mix with the sugar, pressure builds up inside and (occasionally) pops open the lid. I usually release the steam inside before it does this. Can anyone explain why this is happening? What happens to the steam as it get agitated? Thank You!