So I’m wrapping a birthday gift and I grab the scissors to put on the finishing touch: the curly ribbon. (You know – take a length of ribbon, put it against your thumb, press the scissors against both ribbon and thumb and drag the ribbon through.)
The I start to wonder, why does this work? How does this work? Who came up with this, anyway? 
Anyone?
You’re stretching one side of the ribbon and not the other, which makes one side longer, which causes it to curl. I have no idea who thought it up though!
Which side is being stretched? As it goes over the scissors is it the inside or outside (against the blade or not)? Any why is it that if you do it to the wrong side, it get’s REALLY straight?
Anything that has thickness gets unevenly streched when it is bent. Notice how the ouside lane of a running track is longer than the inside lane. If you wrap ribbon around your finger the strech is so small that Since ribbon is very thing it has to be bent at a very tight radius for one side to be stretched. The minute portion of the ribbon wrapped around the edge has the ouside face stretched past it’s eleastic limits so it doesn’t spring back.
This only works with “corrugated” ribbon – you’re clearing out the corrugations on the one side, causing the curl, as explained above.