What causes the vacuum in my bento box? I can't think of a good title for my post, sorry about that. 
I've got a lunch box (called a bento box) Basically it's a plastic box with a plastic lid with a rubber rim around the lid to create an air tight seal with the top is on.
Now when I microwave my lunch (take the top off, stick it in the microwave to heat the contents, then take it back out and put the top on) it creates a vacuum such that sometimes I really need to pry the lid off with a fork. 
How does this work? Does it have to do with the lid and base being different temperatures? 
My very rudimentary understanding of physics is that the hot gases should be expanding and pushing outwards. I would think that would make it easier to open when the contents were hot.
 A: Firstly, we must understand how microwaves work. The microwaves that are used essentially jiggle water molecules. Since most foods have water content in them... then that's why it works.
So water molecules jiggle around and since they are next to other pieces of food, they move too cause they keep bumping into each other. Thus, when all these things move, your food is "hot".
So have you noticed that right after you microwave food, its steaming? That's because the water has gained so much energy that it becomes a gas. Since your lid was not in the microwave and on the counter top presumably, its colder than your food and will thus condense the water vapor because it absorbs the energy from the water. This quick condensing is what I think causes the suction affect to happen.
If you microwave with the lid on, you'll see that your container might explode. Try microwaving food with the lid on and then put it in the freezer right away and see what happens. You'll probably get the suction affect very strongly.
A: The sealing effect is caused by pressure differences.  
Once you put the lid back on, the warm air in the box cools, reducing the gas pressure inside the box below atmospheric pressure.  So the atmospheric pressure outside the box tends to seal the box if there is contact all around the lid to prevent air moving between inside and outside.  Water vapour condensing inside the box adds to the effect.
The same sort of thing can happen when you close the door of a refrigerator and the air inside cools, again leading to a partial seal.    
A: OK this is pretty simple.  When the soup cools the pressure drops below the outside pressure and seals.   That was said already.   But usually a soup container in a traditional bento box has a little push button valve on top that when pressed releases the vacuum within the container.  Be sure to use the right container for your soup or look into getting one with the valve.   The vacuum provides additional insulation once formed so the food stays hot even longer.   The best ones I know will keep soup warm for six hours.   
