I can't figure out how to adjust the size they come in (35mm) to 120. There must be a trick to it, but I can't figure it out. Surely they're not just pulled apart! In any case I've been reluctant to try that for fear of breaking them. There's no directions on the package. It's probably something very simple that I'll kick myself for not figuring it out on my own. Anyone who's used them got any hints?