What causes coolant that was expelled from the system to the expansion tank to be return to the cooling system as the engine cools?

Question

Quizplus · Accepted Answer

The vacuum relief valve in the pressure cap opens when the engine cools down, allowing coolant that was expelled to the expansion tank to be drawn back into the cooling system as a result of the negative pressure created by the contracting of the coolant. This helps ensure that the cooling system remains full and functioning properly.

What Causes Coolant That Was Expelled from the System to the Expansion