What activity do some people like to do all the time — just about anywhere — that involves joyful, vigorous, rhythmic movement? Hint: It relieves tension.

You’re right. The answer is knitting.

It’s something people do in surprisingly large numbers at Harvard, where at least 20 informal knitting circles meet once a week. Students, staffers, and retirees work their needle magic for community, for relaxation, and often for charity.

“In the past 10 years, knitting has just exploded,” said Harvard librarian Rhea Lesage. “And a lot of it has to do with the social networking.”

Lesage belongs to one of two groups that meet once a week at tables outside Lehman Hall. (In bad weather, the knitters head for the Dudley Café.) Another knitting circle meets at Café Gato Rojo, and at least one has woven its way into the fabric of Harvard Law School, Harvard Divinity School, Harvard Neighbors, and Harvard College Library Technical Services.

“The rule is: We never talk about work,” said Lesage, a bibliographer for Modern Greek. Though that rule seems to go against one popular nickname for knitting circles — “stitch ’n’ bitch.”

Knitting has advantages beyond the social. “I started so I wouldn’t snack at night,” said six-year knitter Karen Moore, a Harvard computer support specialist who discovered that she can’t eat when she has a lap full of yarn. “It really worked.”

Lehman group knitter Leon Welch, a purchasing assistant at Harvard University Health Services, worked on a pink, lace-pattern scarf that he said will be “elegant, luxurious, ethereal.” But meanwhile his delicate kid mohair-synthetics blend knotted and broke a lot. Peering at his work in the sun, Welch said, “There are a lot of errors in here.”

He started knitting five years ago when his son, a fifth-grader, was taking knitting as an art elective and taught his father the basic stitches — knit, purl, and cast on. “I was hooked,” said Welch, who now sees knitting as an art practice along the lines of music, calligraphy, or painting.

“Knitting provides a way to use my hands to create something — and there’s no pressure,” said Welch. “It relaxes me a whole lot.”

The psychological benefits are powerful, agreed Lehman knitting circle veteran Jill Young Coelho, a retired Harvard librarian. “For me, knitting is a form of meditation.”

But don’t cross needles with Coelho, who recited her favorite knitting button: “Bad attitude. Pointy sticks.”

Lesage wears a button that says, “I knit so I don’t kill my family.”

Still, part of knitting’s charm is social. “I love talking to people,” said Welch, “and this is a wonderful conversation starter.”

Coelho said that is especially true if you are “K.I.P.,” or “knitting in public,” a staple of the craft’s jargon, which is often couched in acronyms.

Everyone in the Lehman Hall circle agreed that knitting circles attract far more women than men, but that the gender balance is shifting.

“There were a lot of men” at the annual Sock Summit knitting conference in Portland, Ore., last year, said Daryl Boone, a Harvard bibliographer. “And a lot of men are designing now too.”

Still, most of his own male friends remain wary, said Welch. “When I show up with the needles, they go the other way. I don’t understand what the big fear is.”

Knitting is attracting a wider audience these days, and its demographic is getting younger. It even has its own Goth subculture.

So here is an art that is democratic, social, calming, and a two-gender practice, as well as portable.

Lucy Gedrites, a 34-year knitter and a Harvard copyright officer, knits on the subway, in line at the store, on the exercise bike at the gym, while reading a book, and in the car. “My husband is a very aggressive driver,” she said. “So if I knit while he drives we don’t fight as much.”

Knitting offers something else: a final product. “I love the results,” said Boone, a knitter for 45 years. But she repeated the old joke, too, that knitters “know we could buy something we made for about two bucks at Target.”

Materials for one sweater made with fine wool can cost $300, and a single fist-size ball of high-end yard can cost $25. “It’s a money pit,” said Coelho.

Then again, “Knitting is cheaper than therapy,” reads a button Lesage has at home. “It did help me through my son’s teenage years,” she said.

Knitting is also just plain fun. Last spring the Lehman Hall group went on a “Red Line yarn crawl” that started in Dorchester. Along the way, they found a new L.Y.S., a “local yarn store.”

But even knitting has a dark side. For one, said Gedrites, there is the U.F.O., knitting jargon for “unfinished object,” a project that gets left behind.

And there are the mistakes, said Coelho, who described “tinking” (when you have to knit backwards to fix something) and “frogging” (when you have to “rip it, rip it”).

Darkest of all is what Lesage called “the knitter’s dirtiest secret,” the dreaded “stash”: that once-loved yarn that piles up in bags, boxes, and drawers. But even that has a brighter side. After all, a “stash-expanding excursion,” knitters say, merits a happy knitting acronym of its own: S.E.X.

Takemi Fellows take Harvard, tackle international health