I'm not sure how universally this question applies; I live in Canada. When we buy potatoes, they often come in large bags made out of heavy paper. The bags are closed by sewing a string into the top of the bag with a distinctive stitch. Sometimes it is very easy to open a bag: you just pull on the string. Other times, not so easy. Noone seems to know whether the efficacy of such an undertaking depends on the method used. Some suspect or even claim that snipping the string in a certain location, or pulling in a certain direction, will guarantee success.
However, it is also possible that some of these bags are just not sewn correctly. It may also be that any such ease of opening is altogether a side effect of the bag-closing process, and that they are not meant to be easy to open.
Is there a reliable method for opening potato bags with ease and rapidity?