I've made a video showing a trick I came up with (although I'm very sure it's been done before) to remove a stubborn screwtop. Now you can share in the fun.
Because I didn't put a single thought into production - I grabbed C. as she was walking by and said, "Hey! Hold this phone and video this!" - let me spell it out:
• If you're having trouble removing a screwtop from some screwtopped thing—a bottle, jar, steam-cleaner canister, or whatever—get yourself a length of strong string. Thin, normal stringy-string will probably not work. A shoelace might do.
• Tie a loop in the string that will fit snugly over the screwtop, put said loop over the top, and wrap the string around the top a couple of times. (You don't have to make a loop, it just makes it a little easier. If you like, just wrap the string around the top several times.)
• Pull on the string - et voila - it should come off with a good strong pull that even a not-so-strong person could manage. (If you're doing this with a jar of pickles or somesuch, you're of course going to have to be careful about pickle juice flying around, so get your jar good and stable before pulling the string.)
Here's the video:
At the end of the video, the lovely and musical voice you hear is that of the lovely and musical C., saying, "This if for my clothes-steamer," meaning the canister. Just so you know.