Why does the lid stick to the pan after cooking?

[fishingspree2]

Homework Statement

A perfectly fitting pot and its lid often stick after
cooking, and it becomes very difficult to open the lid when
the pot cools down. Explain why this happens and what you
would do to open the lid.

The Attempt at a Solution

I have no idea why. All I know is that atmospheric pressure is applied to the lid and that the vapor inside the pot applies pressure against atmospheric pressure. Also, I have a question: is the vapor subject to atmospheric pressure? I am thinking that since the lid fits perfectly with the pot, the atmospheric pressure is only applied to the lid and not the vapor.

Can anyone help me?
Thank you very much

 

[JaredJames]

I'd be inclined to look at this from the following point of view:

The internal temperature of the pot (even without the lid) is greater than the external temperature. And so the density of the air inside the pot is less than that of the external air (you feel hot air rising above it). When you put the lid on, and you let the contents cool, the internal conditions equalise with the surrounding atmospheric conditions. As the temperature drops, the internal pressure also drops. This lower internal pressure coupled with the atmospheric pressure pushing on the lid...