"It's the new math," said Tom Lehrer, "and only a kid can do it." There's a parallel theory in our city: Metro is for kids -- it's more easily mastered by a 21/2-year-old than a 50-year-old. My rather painful discovery of this truth occurred during some recent research conducted on one of the Monday holidays. Two friends -- linked through a grandmother / granddaughter relationship -- boarded the subway in Ballston for a trip to the museum.

In describing the experiment, the younger -- aged quarter-to-three -- will be called the Small Friend. The elder -- who until that day had a self-image of vim and vigor, slowed only by the need to put on glasses to read the farecard instructions -- will be called the Elder Friend.

The sample is admittedly small for such a general theory. But were the experiment repeated in a larger sample of similarly paired subway riders, I'm sure the disparity between riders' ease of travel would be repeated in most cases. THE EXPERIMENT: Going to the Museum on the Orange Line. STEP ONE: At Farecard Machine Small Friend (after observing the customers in the line ahead): "Grandma, need dower." (This was not in reference to laying by some cash to insure her future marriageability -- she could care less -- but a statement that she needed a leaf of the green stuff to stick into the machine.) "Pig me up." (This is accompanied by the universal gesture of arms raised in sun-worship position.) Height of almost-three-year-olds is a handicap that Metro engineers have not yet dealt with. Pokes all the pushable buttons and ultimately one spits out a farecard. (Saves reading the directions, especially if one is not yet literate.) Meanwhile, Elder Friend: Shifts umbrella-stroller to other arm. Opens pocketbook. Takes out glasses and place on nose. Takes out wallet. Searches for "dower." Hands to Small Friend. Picks up Small Friend to machine height, balancing stroller and purse. Starts to read instructions about how much fare is needed to Museum, which means, let's see, getting off at Federal Triangle, which is still on the Orange Line. Whoops, too late. Small Friend has already bashed the farecard eject-button and grabbed the card. STEP TWO: At the Farecard Gate Small Friend: "Me do it. ME DO IT! ME DO IT!" Takes card and jams it in the slot as she sees others are doing. If it won't go, try again. Try various different ways. Keeps an open mind when Elder Friend suggests the stripe must go on the same side as in the picture. Tries it that way; it works. Wow, it pops out the top. Aha, you get it back again, so grab it. Studies it to make sure it's the same card. Is aware that the gate has opened and closed again. No sweat. Just ducks under because that's where everyone is going and it looks like another escalator just ahead. Meanwhile, Elder Friend: Lets her subconscious wonder how Metro engineers could have made these stations absorb all sounds except the insistent voice of one Small Friend. Permits Small Friend to have farecard, knowing that quarter-to-three is far too young to be able to figure out the machine. But suggests that the ticket goes in with the stripe on the same side as in the picture. Watches Small Friend mimic other passengers by pulling ticket -- virtually out of line of Half-Pint's sight -- from the top. Realizes the gate is opening and that between it and herself is a three-foot-tall immovable midget. Considers leap-frogging the midget. Instead squashes it against the ticket pedestal in an effort to get through the gate. Gets "twanged" as the gate closes in pincer movement. Reaches back to grab Small Friend, who has already wriggled underneath, breezed past with all the nonchalance of an intent commuter, waving the farecard, is saying "Me carry it." Arrives at the ecalator far too late to say the customary, "Now take a BIG step." STEP THREE: On the Subwayi Small Friend: Since the boarding is at the end of the line, there's a choice of seats. Chooses one that faces backward, naturally. At first sits boldly in a whole seat. Announces to Elder Friend and all other passengers, "This a train, Grandma? This a train!" Awe begins to set in. Meanwhile, Elder Friend: Wonders if Small Friend will have any trouble crossing the flashing floor lights and the small gap onto the train. Too late; Greased Lightning is through the door. So takes a seat next to Small Friend, facing backward (yuk!) and studies the map. Assures Small Friend with a nod and a hug that yes, this is a train. Feels the bravado go out of Small Friend as she snuggles under one arm, getting down to the important business of being a passenger on this wild and wonderful thing called Metro. CONCLUSION: Lest the grandparental generation and/or the designers of our city's favorite hole-in-the-ground feel that the subway system has become child's play, there's a reassuring finale to the experiment. After the two friends had seen the elephant, "bumped"down the curving museum stairway when the guard wasn't looking, stood fascinated at the dollhouse and had a cafeteria lunch of chocolate pudding, they rode back to Ballston. In the last process, Small Friend inserted the farecard, stripe in the right direction. Elder Friend stepped deftly through the gate (even grandparents can learn) and, turning to retrieve Small Friend's hand, found utter dismay flooding the little face. "Grandma, it eated my ticket! It didn't give it back!" What are friends for, if not to scoop up small weary people, press them against a shoulder and try not to let the smugness show: "Yes, sweetheart, sometimes the machines do that. Sometimes they just do that.""