Hey everyone. I co-own a car shop and today we were having a discussion on why when you unscrew a hot cars radiator cap that it geysers out. I was saying it's from the fact that the cars system pressurizes the coolant to raise the boiling temp so when you unscrew the cap the boiling point drops to 212 due to the rapid pressure change causing instant boiling and steam to create the geyser. My business partner says "The cap makes it build preasure which raises the boiling point. When you open the cap on a hot car you get instant thermal expansion which then moves the fluid on its path of least resistance". What's the actual reason and science behind why when you pop the radiator cap off a hot car that it will create a fountain of antifreeze?