fun fact: not being able to open a jar usually isn't due to not being strong enough, but rather due to your hands not being grippy enough at that moment, could be due to sweat or due to natural oiliness. if you hold the lid using something extremely grippy (like rubber) then you'll most likely be able to open it, though if you're using something with really high grip (e.g. the inside wall of a tire) it might feel like it's gonna tear your skin off sooner than the lid opening.
in most cases though the problem is that the jar was sealed while warm or that the pressure inside the jar dropped somehow, making it almost impossible to open.