Usually Tom Rosenberg works on another island, Manhattan, on 47th street and sixth avenue. Today, however, he is building a path in Aquinnah, connecting the new Philbin boardwalk to the beach. He chooses his rocks and words carefully. “You want your rocks smooth and round,” he suggests. “Nothing worse than coming onto the beach and stubbing your toe.”

Tom pauses and stares down the beach. It’s a regular building boom. Sand castles with moats, forts and dredges, dams and tunnels, construction sites that defy names and spark the imagination.

Just beyond the path, Max, 22, shows off his latest creation, a rectangular pit with steps leading down and a tunnel leading out. It’s about eight by ten feet and looks a little like a cross between a Mayan temple and a shopping mall. “I love this sand,” he says. “The black stuff’s easy to sculpt with. It doesn’t cave in. And if it does, you start over.” He surveys his kingdom and proclaims, “I like to dig.”

Sometimes at the beach, even when you think you’ve seen it all, you’re struck by genius, like 11-year old twins, Marcus and Spencer, and Trevor, their 13-year old brother. Each is hard at work sculpting balls, a little smaller than the bowling variety. Each boy’s ball is a different size and perfectly round, glistening in the sun.

How do they do it? “You mix up wet sand, cover it with drips and smooth it out,” Marcus patiently explains. As he speaks, he finishes a ball and lines it up next to 15 others. On the pedestal beneath them, in sand, he has drawn the words SAND BALLS, as if it’s a plaque under a painting at the Louvre. “We looked on YouTube and there were only two sand ball clips,” he says proudly.

Bob Kegan, 70, at Lucy Vincent Beach, also sees magic in the drip. He’s a true artist, keen on passing along his lifelong Gaudi-style drip-castle technique to his two grandsons. “Sand-building is like three-dimensional finger-painting,” he says. “You have direct contact with your paint, your hands are your brush, and it’s as much about the process as the result.”

No blueprints, no instruction manuals, no sand castle coaches. Here at the beach, children unburdened by tennis lessons or SAT prep, community service projects. Children free to make messes and not have to clean them up. The tide takes care of all that. And if a turret crumbles or a moat seems iffy, no one seems concerned about how their self-esteem will suffer.

The beach offers the chance to shape raw elements, to create and destroy, to fashion new worlds in which we are the rulers, to play God. Just stroll down any Vineyard beach on a random day in summer and you might spot one of these builders and hear what they have to say.

Dublin, age nine, and Trinity, 12, are making a pool because “it looks nice,” and then scattering rocks into it with a spoon.

Ron, 30 is putting the finishing touches on the Eiffel Tower.

Marshal, age three, at Menemsha Pond, describes his lava flow.

Emily, also three, when asked her favorite thing to do at the beach replies: “I like to bury Barbie in the sand.” Her mother, horrified, insists: “You just do it up to her neck.” Emily shakes her head in disagreement.

But not to worry. Chris Rasmussen, a teacher at the Vineyard Montessori School, points out that destroying doesn’t mean the same to children as it does to adults. Barbie may be buried but, mercifully she can always be disinterred.

Chris echoes Bob Kegan’s observation that sand-building is more about process than product and says that for two-year olds digging in the sand is all about filling and dumping, filling and dumping. “Repetitive action is joyful. There’s this great Ah-ha moment, I can do it again.” Her advice to parents is not to label their child’s sandy creation or tell them how beautiful it is. Instead ask them to tell you their story about it.

John Abrams, of South Mountain Construction, may be a bigger builder now than he was as a child at Shell Beach in Inverness, Calif. But he’s still as enthusiastic and loves to tell about it. “In the old days, you didn’t have stuff,” he says. Asked if he ever played in the sand, suddenly he seemed a kid again. “Oh my God, we made mountains out of sand. We carved roads with tennis balls rolling through them like those marble games. We dug holes and tunnels and built bridges close enough to shore so water would fill the cavities, but not wash them away.”

While we’re talking about sand, just for the fun of it, count how many grains are in a teaspoon (a magnifying glass might come in handy). Then multiply that number by all the deserts and beaches in the world, and — hold on for a second — the Earth has roughly (very roughly ) 7.5 x 1,018 grains of sand, or seven quintillion, five hundred quadrillion grains.

That’s lots of castles.

But how do we come up with sand in the first place? One possibility is that a glacier crushes a rock into sand, then it is carried into rivers, slowly by mudflow, slump or creep, and, finally, continues to the sea. Waves break it down and round it into smaller and smaller particles.

Then — my favorite part — after beach sand is deposited offshore (way before tax-free accounts,), millions of years go by and our friend, sand, turns into rock and ultimately becomes sand stone. Then, hundreds of millions of years later, or just maybe because two continental plates happen to collide, it’s drawn down into the Earth where it eventually melts and becomes magna.

All this is of no interest to 10-year old Maeze. “I’m building a big house,” she informs, pointing out its features like an enthusiastic realtor on HGTV. “It has an indoor pool and a water slide.” Her brother, Thomas, 12, is a more ambitious developer. His project is “a huge kingdom with big castles and standby houses.” A standby house? “It’s part of the kingdom, but not within its walls,” he explains patiently.

At the end of a summer afternoon, Vineyard parents stand up and shake the sand from their towels. “Time to go,” they call out. “Five more minutes,” children beg. A compromise is reached, and soon all trudge to the parking lot.

Here, where land meets sea, the beach remains littered with sandy ghost towns, soon to be flattened and vanish with the tides. We take one last look and realize we’re not on such firm ground as we may think we are, nor will our creations necessarily be lasting ones.

In the morning, fresh opportunities beckon.

Ted Sutton lives in Newton and Aquinnah.