It turns out that our grandmas have a number of different hacks to open stuck jars, which explains why you never saw her asking your grandfather to open one for her. In fact, she used rubber to her benefit when she found herself face-to-face with a stubborn jar, and one of her favorite old-school hacks was to use a rubber band. She then placed the rubber band around the lid of the jar.
By holding onto this rubber band rather than the slippery jar lid itself, you can actually get some grip and open it once and for all. And it's a foolproof method.