So, why does ribbon curl, anyway? (or, the wonders of internet search engines)

The Boy found a piece of ribbon today from a Christmas present and was playing with it, pulling it through his fingers and scraping one edge over his thumbnail to make it curl up.

"Why does it do that?", he asked.

Well, I have no idea, but I know how to find out.

I did several searches, and found a few answers ... but I wanted an answer from a reputable source ... I mean, I wouldn't want to be passing on inaccurate information, now, would I?

Well, fellow enquirers, we are in luck: Scientific American has an article posted about this very subject. (I admit that SciAm is not exactly a peer-reviewed journal, but for our present purposes, it'll do).

A Harvard physicist named Buddhapriya Chakrabarti actually did experiments on the optimal method for curling ribbon:

The popular belief is that pulling faster and with more pressure yields tighter
loops, Chakrabarti says, but their experiments proved that "if you hold the
tension constant and if you make it go slower, it curls even more." More
pressure, in the form of heavier weights, did not tighten the curls, he adds.
The researchers found that the pressure only had to exceed a certain threshold,
which they are set to report in March at the annual conference of the American
Physical Society in Denver.

Chakrabarti says the ribbon curls because its outer layer stretches and, therefore, expands, more than the inner layer that is pressed against the rod or scissors. "Even when you're doing it with a pair of scissors, it's not absolutely flat—you're not pulling it flat," he says. Putting the ribbon on a table, for example, and rubbing the scissors across it does not work very well, he says. (Not to mention the possibility of damaging the table.)

The ribbon must also be taut, Chakrabarti points out, possibly so the molecules in the plastic get pulled apart. He notes that whatever the microscopic details are, pulling slower allows the plastic to relax into a curly state, because it cannot easily snap back into its old, flat one. Do not try the scissors method on satin strands, he says, because it will not work. The reason: stretching does not break the material down in the same way, because satin is woven and not a continuous sheet.

There you go. Another mystery solved.