When you see a penny on the street, do you pick it up? Probably not. Why bother? They're worthless. They exist only to satisfy sales tax. I have several jars of pennies I use as bookends. The Treasury Department is seriously considering discontinuing the penny, forcing prices to be rounded to the nearest nickel.

But 40 years ago, pennies were real money. For just five, I could run down to my corner store and buy a fat Hershey bar loaded with almonds. Based on their dates and mint marks, some were worth a great deal more than their face value, especially to the gang of young teenage boys I ran with. We argued over the wear on Lincoln's cheekbones and other grading details that could distinguish a penny worth three cents from one that might be worth up to 25, or more.

I grew up in the 1960s in a Long Island suburb of New York City. My junior high Coin Club had two dozen members, mostly kids with Newsday newspaper routes who scanned the change they were paid. We collected pennies, nickels, dimes and quarters, but pennies were by far the most popular.

As a 12-year-old, it was thrilling to discover coins in the change that I knew had collector value many times their face value. We coin collectors were a secret society, possessors of uncommon knowledge.

One summer day at Long Beach, a 50-cent ride from home on the Long Island Railroad, I bought a Good Humor bar and received in change a 1938-S penny - S meaning it was minted at the Old Mint in San Francisco - in Very Fine condition, an official grading designation. I knew it was worth a good 25 cents. So did my buddies, who oohed and ahhed.

The social studies teacher who sponsored the Coin Club, Mr. Hewlett, used coins as a window into American history. Why were so many coins from the early 1930s valuable? Because during the Depression, production was unusually low. I'd heard tales of the Depression, of course, but somehow the value of the coins from that period made it real for me. Mr. Hewlett also regaled us with tidbits of numismatic history, such as tales of the Old San Francisco Mint.

At Coin Club meetings, we swapped coins based on the value listed in annual price guides. My best friend, Ira, was a Coin Club celebrity because he owned a beautiful specimen of one of the rarest Lincoln cents, 1909-S. He bought it from some rube for a mere $5 when it was worth about $15. During that year, the Old Mint struck only 1.8 million 1909-S pennies, fewer than the Denver and Philadelphia mints now strike in one day.

Compared with its sister mints, San Francisco's was small, and had lower production. As a result, the most collectible coins almost every year had S mint marks. In addition, few S coins made it back East, which, in the world of my New York youth, made them even rarer and more valuable.

My girlfriend, now wife, Anne, and I moved to San Francisco in the summer of 1975 so she could go to medical school at UCSF. On the drive out, I mentioned that I wanted to see the Old Mint Museum. Anne was vaguely aware that I'd collected coins as a kid, but thought it was ancient history. Recalling Mr. Hewlett, I explained that the Old Mint was one of only three buildings to survive the 1906 earthquake. It was called the Granite Lady because of its stately stone construction, and the Fort Knox of the West, because it once held one-third of the nation's gold reserve. The museum displayed a collection of coins that had been struck there. Anne was incredulous. "You want to see old coins?" Her expression said: Who is this weirdo, and how did I wind up with him? "Go by yourself."

One morning during that first summer, shortly after Anne and I settled into a sublet on Dolores near the New Mint, I hopped the J-Church and spent an afternoon at the old one at Fifth and Mission streets. The collection was disappointing, small and poorly displayed, and the building felt dilapidated. Nonetheless, I had a ball.

In 1994, when the Old Mint Museum closed and the Granite Lady was shuttered,

I felt bereaved. How could San Francisco forsake one of its oldest and most storied monuments? Now, fortunately, the building has been awarded to the San Francisco Historical Society, which hopes to reopen it as a City Museum on April 18, 2006, the 100th anniversary of the Big One. With rehabilitation costs estimated at $50 million, success is by no means assured. But I'm pulling for the project. To me, the Old Mint is as much a symbol of San Francisco as the Golden Gate Bridge - but with much more history.

Every night before bed, I empty my pockets, cull the pennies from my change and check them. At 53, I have to use a magnifying glass. These days, I rarely find any S mint marks. The Mint stopped striking them in 1974. But every so often, one turns up. I place it in a special jar, and for a moment, I'm 12 years old again, thrilled to discover buried treasure in my pocket.