Re: Sometimes when you heat soup in a sealed tupperware container the lid will

    That's a neat question, and first I'll give you a hint -- is the soup 
the only thing being heated?









The reason that it sometimes collapses a bit first is because, depending 
on the thickness of the walls and the size and shape of the container, 
sometimes the tupperware itself expands faster than its contents. Not for 
long, admittedly; the expansion of the soup, especially if it starts 
boiling, becomes much, much greater than the plastic.