If you're trying to embellish the gift for your sweetheart, cut the ribbons using a pair of scissors moving firmly but slowly to get the best curls.

"Conventional wisdom supposes that a quick scrape of the blade makes for tight curls, but slowing things down seems to give the ribbon more time to adjust to its new, curly state," says physicist Buddhapriya Chakrabarti of Harvard University, part of a team that tried different curling techniques. "I wanted to test whether the common belief is actually correct," says Chakrabarti, who made a systematic approach on the issue.

So he and his team, including an undergraduate student, Anna Klales, from Haverford College in Pennsylvania, developed a motorized curling device. They tagged thick plastic ribbons (like that employed for transparencies) to the motor and draped them over a metal stem, stretching them by hanging a weight from the free end and the motor pulled them across the stem. "The popular belief is that pulling faster and with more pressure yields tighter loops, but their experiments proved that if you hold the tension constant and if you make it go slower, it curls even more", said Chakrabarti. "More pressure, in the form of heavier weights, did not tighten the curls," he adds.

The measurements revealed that the pressure only had to go over a certain limit. "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," said Chakrabarti. "Putting the ribbon on a table, for example, and rubbing the scissors across it does not work very well. The ribbon must also be taut, possibly so the molecules in the plastic get pulled apart." he added.

Whatever the molecular structure of the plastic might be, pulling at a slower pace permits the plastic to relax into a curly state, because it is hard to re-adjust into a flat structure. "Do not try the scissors method on satin strands, because it will not work", he says.

But stretching will not have the same effect in satin, which is woven, but does not have a continuous structure.