A little Valentine's Day tip: if you want to curl ribbons using a pair of scissors, researchers say the secret is to be firm but slow. 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 tested different curling methods.

"I wanted to test whether the common belief is actually correct," says Chakrabarti, who started by putting scissors to ribbon but quickly found he needed a more systematic approach. So he gathered together some colleagues and enlisted an undergraduate student, Anna Klales of Haverford College in Pennsylvania, to assemble a motorized curling device. They attached thick plastic ribbons (similar to the plastic used for transparencies) to the motor and draped them over a metal rod. They stretched the ribbons taut by hanging a weight from the free end and used the motor to pull them across the rod.

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.