A good sidewalk chalk drawing is like a good creme brulee. It takes hours of work to get the color, texture and balance perfect. And when that's finally done, you have a few minutes, maybe, to step back and admire the result before it's gone: absorbed, digested, a fragment of memory.

A giant Edward Hopper painting lying flat on 17th Street NW had the same destiny last week. Caroline Irons, a 24-year-old painter employed to re-create masterpieces on the sidewalk outside the Corcoran Museum of Art, drew a nearly perfect imitation of Hopper's "Ground Swell," down to the angle of the sun on the crests of the waves. But then a child threw the contents of his sippy cup all over it, washing away her sailboat's sail and making the ocean run red with Kool-Aid. Oh well, she says. By now, she's used to people walking, jogging, biking and spilling over her work. It is made of chalk, after all -- 15 colors of Crayola, at least six of which are perpetually smudged on her forehead and feet.

Irons is paid $10 an hour to take a medium usually reserved for children and use it to copy the masterpieces that hang on the Corcoran's walls. All day long, she gets confused and admiring looks. Kids stare as if she's decided to build a luxury car out of Legos. Adults dodge the drawings on their way to work. Art lovers shake their heads: All that talent, and it's going to disappear the next time it rains.

"Yeah, they wash away," Irons says, "and people wear them out walking back and forth during their lunch break. But when they're gone, then you have more room to do work."

In the last two decades, sidewalk chalking has become fashionable in the art world. There are festivals everywhere from Italy to Ohio that draw painters who create museum-quality works in parking lots and on blocked-off streets. Crayola sells 70 million pieces of sidewalk chalk a year, more in the summer than any other time. They come packaged in little plastic tubs, one of which Irons uses to collect tips from people who are bewildered or impressed by her combination of high skill and low form.

If a sidewalk drawing can be a masterpiece, then maybe anything -- an expense report, a grocery list, a doodle -- can be one, too. In that case, everyday life provides countless opportunities for individual greatness. Walk the dog like Baryshnikov. Or mow the lawn like Frederick Law Olmsted.

"We as people create art all the time that isn't meant to last: a gourmet meal, for example, or a flower arrangement," says Cleveland-based chalk artist Robin VanLear. Sidewalk chalking "has this sort of sense of weather and being there and welcoming people over a period of time." A work's impermanence can actually be the source of its artistic value.

The history of this seemingly innocent pastime is in fact remarkably grim. Beginning in the 16th century, impoverished soldiers and amputees would perch on the steps of churches and beg for money. This was the place to go because markets were usually set up across the street, and in the shadow of God's house, shoppers were more likely to hand over spare coins to indigents. At some point, VanLear says, someone decided to court alms with art, found a piece of charcoal and drew a picture of the Virgin Mary.

This, apparently, proved lucrative. Others followed, eventually incorporating color into the drawings to attract more attention. Usually, VanLear says, they copied the Madonna and Child paintings of Raphael, the most popular Renaissance master at the time. Their style, which was more Mark Trail and less Michelangelo, came to be known as I Madonnari, meaning "painter of the Madonna." It spread through Europe in the centuries that followed, but until the 1980s, when American artist Kurt Wenner and his followers elevated sidewalk chalk into the highbrow, it remained what it was: a sophisticated way of begging for money.

Dick Van Dyke introduced a lot of Americans to the powers of sidewalk chalk almost two decades earlier in the 1964 movie "Mary Poppins." His character's drawing of a bridge led to a tea party with Mary Poppins, complete with penguin wait staff, and an afternoon fox hunt with a poorly constructed merry-go-round. But then came an English downpour to end the fantasy. The cartoon world melted away and Jane and Michael went home to bed. Herein is the central genius and problem of sidewalk chalk.

Good for a sunny day but bad for yesterday, when sporadic showers interrupted 30 elementary schoolers' efforts to paint flowers and school buses and themselves across one block of North Hudson Street in Arlington. Part of the Museum of Modern ARF's Blue Moon celebration, this sidewalk chalk painting festival was organized by artist and museum director John Aaron. The kids seemed to mind the rain, running around with big black trash bags in an effort to preserve their work and their skin, which was alternately purple or green or yellow, depending on the color they had chosen to paint themselves. The professional artists, including Claudia Olivos, who drew a giant, grinning sun against the museum's brick wall, were less concerned.

Aaron works in porcelain, a medium, he says, that if not dropped will last forever. "That's probably why I get a kick out of drawing something that disappears," he says. "It keeps me humble."

Rex Weil, a professor of art theory at Corcoran College, says it can be headier than that. He says that sidewalk chalking at the professional level is more performance than visual art. He has a lot of complicated ideas about how this developed, including that it is a rebellion of contemporary artists against the capitalism of modern art-collecting. Alternatively, he suggests, "it's a critical posture, a kind of submission to forces that are just undeniable, which are basic entropy. Life and materials are eroding, burning up right as we speak. That's mortality."

Irons was producing another example of mortality last Thursday afternoon, a "Young Woman in Kimono," taken from Alfred Maurer's 1901 painting. Her concern was the development and deterioration of human attributes. For hours, as she works, the details of the young woman grow and sharpen. A smear of chalk becomes a sleeve becomes the silk collar of a red and green kimono. The woman's skin is white, then flush, then tan and lifelike as Irons goes over and over it in the afternoon sun. But with time, the painting ages. First to go are the flesh tones. When the clothing is brightest, the facial features have softened and faded. After a while, the picture is barely visible. If you look closely, you can pick out a nose or a hem or maybe the line of the woman's jaw. Eventually, it will be just wet pavement.

"In the mornings I'll touch up the bike treads, but otherwise, I just let it disappear," she says. This is what she's done with each of the 20 copies she has chalked this summer. Fix what you can, but ultimately you have to let nature take over.

Caroline Irons puts the Mona Lisa on an Arlington sidewalk, where it will be on view until the next rain. Artists show their skills with chalk in Arlington as part of the Museum of Modern ARF's Blue Moon celebration. Above, Nora O'Leary admires her work, some of which she winds up wearing, as this is the nature of sidewalk chalk drawing.