ROBERT CROLL, who turns 10 on July 4, has seen a lot of flags in his day. After all, "he's a Yankee Doodle Dandy," his father, John, says. But when the Crolls of Cupertino, Calif., recently visited the National Museum of American History in Washington, Robert saw the granddaddy of them all: The Star-Spangled Banner. That is the flag Robert will be thinking about on Sunday.

Robert is in good company in recognizing the historical significance of the 30-by-34-foot banner. Last July, President Clinton drew attention to a major effort to preserve the original flag that flew at Fort McHenry during the War of 1812 and inspired the Francis Scott Key poem that later became our national anthem.

"If you remember the words and then look at this massive flag," Clinton told a gathering at the American History Museum, "you can imagine what it must have been like in 1814, waving gallantly during the fight."

Today's flag looks very different from the one Key saw 185 years ago; it suffers from decades of exposure to light, dust, humidity and air pollution. Now housed in a customized conservation laboratory, the flag is the focus of an American History exhibition on its preservation and a series of guided science experiments. These activities allow families to transport themselves back to the Battle of Baltimore and step into the shoes of the museum conservators who will spend the next three years examining, treating and preserving the deteriorating banner.

A series of activities, including touching a bomb fragment and textile reproductions and listening to a recording, help families understand the flag's life story. Seeing a 13-inch bombshell -- the type fired at Fort McHenry during the war -- "gives the kids a tangible connection to the words in the song," says Jane Spicer of Columbia, who brought sons Calum, 9, and Graham, 7, to see the flag last month.

Downstairs in the Hands-on Science Center, visitors can participate in experiments to help them understand conservation work. After staff members pass out pink and green protective glasses, petri dishes and microscopes are pulled out.

"Hey, have you guys been to a ballgame lately or a Memorial Day concert?" museum educator Heidi Olesch asks her junior scientists. "What did you sing?"

A discussion of Key's poem leads them to three experiments, including a test to identify fibers in sample swatches. Do the threads look like a twisted ribbon or have scales like a fish? The answer provides clues to whether the samples are made of cotton or wool.

Conservation is like detective work, and there are many mysteries to solve, Olesch tells her group. Determining the appropriate treatment for the flag, 12-year-old Sara Croll says, "is like getting an assignment that tells you how to do a school project -- like a report on the Civil War. . . . You know it's going to have 11 pictures and be 35 pages long, but you don't know where to begin."

After four years of examination and extensive research to determine the condition of the flag, conservation staff have overcome that intimidating first step. Through a 50-foot, floor-to-ceiling glass window, visitors follow the progress to the next step, treatment, as a team of conservators use eye surgery scissors and other tools to separate the flag from its linen backing, which was applied in 1914 using 1.7 million stitches.

"Why are the conservators wearing masks?" a visitor asks chief conservator Suzanne Thomassen-Krauss. If the conservators have a cold, "you don't want the bacteria landing on the flag," she said. You also don't want the staff breathing any airborne microscopic dust. "You want to protect both the flag and the person working on the flag," she said.

This weekend -- from sea to shining sea -- in Independence Day parades, on front porches and at municipal buildings, Old Glory rules. "I think seeing the big one has given new meaning to all the smaller flags I've seen," says Robert Croll, the birthday boy. This year on the Fourth, "I'll think about what the older flags have been through, and I'll think the new flags are fluttering out in honor of all the flags before them."

VISITING THE FLAG

As many as 100,000 people are expected to visit the Museum of American History this weekend, so it may not be the best time to view the original Star-Spangled Banner. A weekday morning is a better bet. The Science Center is closed Mondays and after 5 p.m.

Learn more about the flag before visiting, suggests museum educator Julia Forbes. "Knowing some fun facts before a visit might provide children a frame of reference which would help them understand more of the exhibition," she says. For instance:

The flag originally measured 30 feet by 42 feet, about a quarter of the size of a modern basketball court. Pieces of the flag were probably given away as souvenirs before it came to the Smithsonian.

The flag has 15 stripes, one for each of the 15 states at the time it was sewn, rather than the 13 stripes in today's design.

A single star is almost twice as large as your head.

The red "A" on one of the white stripes probably stands for Armistad, the name of the commander at Fort McHenry.

"The Star-Spangled Banner" is sung to the melody of "To Anacreon in Heaven," a popular English pub song from the early 19th century.

Baltimore flag-maker Mary Pickersgill, who along with her 13-year-old daughter sewed the flag, was paid $405.90 for her work.

THE STAR-SPANGLED BANNER -- The conservation lab and accompanying exhibition will be open for the next three years, then the flag will be permanently displayed elsewhere in the National Museum of American History, 14th and Constitution Ave. NW (Metro: Smithsonian). 202/357-2700. TDD: 202/357-1729. Web site: www.si.edu/nmah. Open daily from 10 to 7:30 through Sept. 6; 10 to 5:30 after Labor Day. Free timed-entry passes to the exhibition are distributed at 10 a.m. (for entry between 10:30 and 1 p.m.) and 1 p.m. (for entry between 1:30 and 5 p.m.). Passes are not required for evenings. Advance passes are available through Ticketmaster 202/432-7328 or 703/573-7328. ($3.50 service charge per order.) The Hands-on Science Center is open to visitors 5 and older. Children under 13 must be accompanied by an adult. The Center is open Tuesday-Sunday from 10 to 5 through Aug. 23. (12:30 to 5 on July 7.) Free tickets, available at the Center, are required. The museum will host a family day on Sept. 18 in celebration of the anniversary of the Battle of Baltimore and the writing of Key's poem.

Other flag resources:

FORT McHENRY NATIONAL MONUMENT AND HISTORIC SHRINE -- At the end of East Fort Avenue, Baltimore. 410/962-4290. Web site: www.nps.gov/fomc. Open daily from 8 to 8 through Aug. 23; from 8 to 7 through Labor Day. After Labor Day, hours are 8 to 5. Exhibits of historical and military memorabilia interpret the fort's history. The visitor center screens a 15-minute film, "The Defense of Fort McHenry," and the restored barracks are open for exploring. Many special events for families are planned this summer, including daily hands-on activities, programs led by Fort McHenry Guardmembers and evening Tattoo ceremonies. A Star-Spangled Weekend with an array of activities will be held Sept. 10-12. Call for a complete schedule of events. $5 for adults 17 and older; under 17 free.

MARYLAND HISTORICAL SOCIETY -- 201 West Monument St., Baltimore. 410/685-3750. Web site: www.mdhs.org. Home of the original "Star-Spangled Banner" manuscript. Open Tuesday through Friday 10 to 5, Saturdays 9 to 5, Sundays 11 to 5. $4 adults; $3 seniors, children 12-17 and students with IDs. Free for kids under 12, and free for everyone on Sundays. Family rate is $6 for two adults and two children.

STAR-SPANGLED BANNER FLAG HOUSE AND 1812 MUSEUM -- 844 E. Pratt St., Baltimore. 410/837-1793. Web site: www.flaghouse.org. Open Tuesday through Saturday 10 to 4. Home of Baltimore flag-maker Mary Pickersgill. The adjacent visitors center houses the 1812 Museum. On July 3, the museum offers "Celebrate the 4th on the 3rd" from noon to 4 with numerous activities and refreshments for families. A "Fifty Fabulous States" program will be held Aug. 21 from noon to 4. $4 adults; $3 seniors 65 and older; $2 children and students.