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? 
Heh, this used to trip me up with feed bags all the time...  The bags are machine-sewn (of course). For each stitch, a needle pushes the string through the bag creating a loop which intersects the loop from the previous stitch on the other side. Pull from one end, you'll pull the loop out all the way across. Pull from the other, you'll pull it tight.   Hold the bag to where you're looking at the "messy" side of the stitches (with all the interlocking loops). Find the edge where the loops start - the end of the string will probably be tucked into the first stitch. Pull it out, and keep pulling...  Steps provided by an anonymous reader:   Loosen the loops of the "messy part" up to the edge of the bag, where the real sew actually begins.  Then, pull gently on the "clean" side of the first chain of the sew (the side that has 1 string, not the one with the loop).  And there you go all the way!   