Many jars have screw-cap lids so tight opening them with bare hands is near impossible, even using a gripper cloth or pad. Wrestling with them near your knees to get more force isn't a good idea if the contents are liquid - olives in brine, for example, in case you suddenly succeed! I know you can buy various clamp-type gadgets, but nothing is easier than the following method: with one hand hold the jar firmly on a counter top; take a wooden rolling pin in the other hand and tap the SIDE (edge) of the lid all the way around as you turn the jar bit by bit with the free hand. No need to hit it hard. Then, as if by magic, the cap will unscrew easily. It works with all sizes of lid.