Sie Deen and I were trying to open a jar of pasta sauce whose lid was on really tight last night. And I mean, tight like I've never encountered before. It was hard to get a grip on it, but I did once and I felt like I almost threw out my shoulder as a tried to twist it open. This is the first time in my adult life I've encountered an object I wanted to open where all my physical strength was to no avail.
We eventually decided that we were not going to be able to open it with brute force alone. Sie Deen went looking for something rubber to grip the jar with, but I started thinking about physics. If I could heat the top of the jar, theoretically it would expand relative to the glass which would heat more slowly, insulated as it was by the jar top itself. The stove fire was kind of dangerous, but luckily because of my aforementioned wife I have another useful tool around the house for heating things up.
So I called Sie Deen to get her hair dryer, plugged it in, and shot it at the top of the jar for forty seconds. After the cap was hot, I twisted it off quite easily. In addition to loosening the lid's grip on the glass, the hair dryer had also evaporated all the condensation from the cold metal, so it was of twofold benefit.
Next time you need to open a difficult to open jar, pull out that hair dryer.