On 2013-03-15 21:15:52 +0000, Alan Braggins said:

More seriously, was the lunch warm when it was put in and created a partial
vacuum when it cooled? In which case put it in hot water for a while?
(Might even help if that wasn't the original problem.)

Yes it is for hot food. Good plan - I'll try that. I can't believe the
top is cross-threaded as it's on square.

E.