Thirty-some years ago, I learned a very effective trick for loosening recalcitrant jar lids (though since growing into my adult strength, I don't need to use it often).
Hold the jar upside down, and firmly smack the lid, *flat*, on a smooth, solid surface (counter top works well). Works well over 99% of the time. One does have to be careful not to hit at an angle, as doing so can break the glass. How does it work? The rubber or plastic seal in the lid compresses a little, taking the pressure off the threads or cams that hold the lid shut. The seal doesn't rebound completely or instantly, so you have a few seconds in which you can remove the lid with relative ease.
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