Take the tour ×  I had a vegetable soup in a crystal tupperware
and I put it in the microwave to heat it up. The funny thing is
that when I took it out, it was impossible to open it up. The
lid is bent towards the inside, as if the air has escaped the
tupperware somehow.  Can anyone tell me why this might have
happened?   share|improve this question add comment  1 Answer
up vote 1 down vote accepted  When heated, air expands. Now when
you take the tupperware out of the microwave and it cools down,
the air inside cools down again and contracts, and 'sucks' the
lid inwards.  Actually, that's not quite right - it's more
precise to say the air outside presses the lid shut. The bodies
of air outside and inside seek to equalize their pressure, and
normally the tupperware would just shrink / contract until it's
inside pressure is equal to the room pressure. But since the
container is somewhat rigid, it can't shrink enough and there
remains a pressure difference. This causes a force acting on the
lid, keeping it shu