Dear Customer (name blocked for privacy),

I have had this question before.

One way is to stick the whole jar in the freezer after generously apply dish soap to the joint. A little water ran dish soap (just enough water to make the soap slippery, maybe a few drops) will lubricate the joint and inject some moister.

Then place it in the freeze where all of the glass parts will freeze. The moister of the water and soap will freeze, expanding the joint slightly. After several hours, (I recommend overnight) the jar and lid should separate. Make sure to place the jar in the freezer upside down. If you place it right side up, the lid may get stuck deeper in to the jar.

Some people recommend heating the jar or lid. This sometimes work. But remember, the heat is expanding one part of the jar; the heat transfers, and though you may get a widening of the joint enough to get it apart, this does not always work. It is better to freeze the whole thing, so that everything condenses.