I know high pressure allows us to boil vegetables more quickly, and this is obvious when a pan lid is fitted tightly. But I can't figure out why the pressure is still increased when the lid only partially covers the pan, leaving a 1 or 2-centimetre gap. It seems to me that air and steam can pass easily through this space and circulate freely, yet even a partially covered pan of liquid seems to boil more quickly. Why?
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