easy way to understand the cooling system. (assuming everyone here went to school till class 6 and paid attention in class rather than eyeing the girls)
we know that water boils at 100C in normal atm pressure - yep
water boiling forms bubbles - yep
we also know that water is an excellent heat conductor - yep
air = bad heat conductor = bubbles are bad
so that understood - lets take cooking paya at home - you go to the qasai and bargain for 2 "bhains ka paya" - and cook them in a pressure cooker (intereshting)
the reason we use a pressure cooker is to raise the boiling point in it - we do that by raising its environment pressure... Good solid water contact with the paya results in good tasty paya - garnish with garam masala and enjoy with qulcha.
now back to cars.
the cap on the radiator is our pressure regulator - just like the regulators on your kitchen's pressure cooker - it has a thick spring on it - test it by pressing the whole cap in your fist - the big spring is whats regulating the pressure marked on the cap. Raise pressure to raise boiling point and keep it efficient. ERGO - just like you wont open the paya pressure cooker while its on the stove (no matter how hungry you are) - DO NOT EVER OPEN the radiator cap when the system is hot and the rubber pipes are hard. You will seriously hurt yourself - 3rd degree burns are not fun to deal with and the hospital bill would be insane.
We have another problem in the car's cooling system - its almost always overbuilt by at least 40% - e.g. the cars I drive take 10 litres of coolant - it would take a mighty long time to heat that 10 litres up - so we install a gate in it which will be a fully mechanical self sensing unit and we will call it the thermostat valve - it blocks the water and forces it to heat up very fast then release it and then heats up the next batch of coolant and over and over (the valve regulates the flow automatically by varying its opening)
The thermostat valve regulates the coolant flow to keep a set temperature. (it cannot build pressure) - the pressure is built by the expanding forces of the liquid water in the system - when you heat up water it will expand.