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The way sand and water work together shapes our landscapes, and our buildings, and makes the best plaything ever invented.

And at times, it’s kind of magical. 

For example, here’s how you make a drip tower for a sand castle: You dig a hole low enough on the beach that water pours in as you take sand out. You stir up sand and water in the hole. Using both hands, scoop up some of the sand and water slurry, take it to the sand castle, and then let the mixture of sand and water drip slowly through your fingers. When the slurry leaves your hands, it’s a liquid, but as soon as it hits the drier sand of the castle, it’ll become a solid, preserved in the splattery form it made just after landing. Keep it up and you can make a tapered spire of curvy drips.

Sand is like no other material. Dry sand will run smoothly through your fingers. As you add water, the sand will clump together in your hand. Add more water and the lump of sand in your hand begins to jiggle like jelly when you shake it. Add still more and the sand will flow through your fingers again.

The reason behind this is that water molecules have a tiny electric charge to them that makes them attracted to many surfaces. When you have two such surfaces close together, water will fill the space between them and hold them there. To see an example, use paper. Two pieces of dry paper will not stick together, but two pieces of wet paper will. In the same way, water fills the tiny gaps between sand grains, holding them all together.

However, if you put both pieces of wet paper into a bowl of water, they’ll come apart again. Instead of cementing the two pieces of paper together, the water molecules clump with each other and the papers come apart.

So on a sandy beach you have a surface that sometimes acts like a solid — you can walk on it — and sometimes seems more like a liquid. As you walk, you send ripples of pressure though the surrounding sand, causing clams to realize you are there, and to retreat into their burrows with a squirt of their siphons.

Sand is usually on the move. Geologists have charts of our coastlines showing the directions that sand goes in, pouring out of crumbling bluffs, rolling down rivers, flowing along shorelines, piling into peninsulas or dunes. Just as we can divide the landscape into different watersheds, geologists can map out different areas according to the source of the sediment flowing through them. They call them littoral cells, or drift cells.

Creatures who live on sand have to be able to move, to dig themselves in and dig themselves out. Even the most sedentary of sand species — the sand dollar —moves around. It moves its spines in ripples to propel itself across and through the sand. It takes in sand particles to weigh one end down in the sand, tilting the other end up to capture plankton from the water.

And there’s a lot to enjoy about sand. There’s the simple sensory pleasure of feeling your toes sink into the soft, warm surface of the beach. The soft, cushioned surface gives people throwing balls or Frisbees the freedom to jump higher and lunge with abandon. Many runners like to train on sand because it strengthens their ankles and calves. 

Using hands or simple tools, people can make sand into fantastic shapes, decorate it with shells, rocks and seaweed, and channel water to make canals. 

To get a sense of the scope of sand and water to fascinate, stop by the south end of Seattle’s Golden Gardens Beach on a weekend day when it isn’t raining much. This is where a small creek comes out of a culvert and curves across the sand into Shilshole Bay.

There will be kids there, playing with the sand and water. And I mean kids in the broadest sense of the term. You can find preschoolers there, and you can find teenagers, gleefully shoveling away. My eldest is now a six-foot high school sophomore, but he shows no sign of outgrowing this.

The most popular activity is to dam the stream. This usually takes several kids shoveling sand into the stream path, until the water below the dam is a trickle and the stream behind swells into a big pool. As the pool swells, the kids patrol the expanding dam, raising it and repairing weak spots. The bigger the dam, the more frequently the leaks keep coming, until finally a large section liquefies and gives way, releasing an unstoppable surge of water. 

This is the best part: a churning tableau of destruction. When it’s done, the kids will start building a dam again.

On hot, crowded days, there can be more than one project going on at the stream. One group can be building a dam, while another is directing water into a canal system. The kids have to negotiate with one another so that each group has enough water. There’s often a debate among the dam makers between those who rush to repair every leak and those who just want the final destruction to come.

They are so busy they barely notice they are getting wet, and their arms, legs, clothes and hair acquire a dusting of sand. 

I’ve done an awkward dance at the water’s edge many times. Leaving footwear just beyond the waterline, I walk into the water, clutching a towel. In the water, the sand drops off my feet and ankles. I walk out over the wet sand, put my right foot into a shoe, dry my left foot with the towel, then slip my right foot out and dry it. This gets most of the sand off. But then I have to cross a beach full of soft, dry sand, and inevitably some will find a damp spot and cling there. So I brush it off, but by now the towel itself is quite damp and sandy, so inevitably there’s a feeling of grit.

Luckily, I don’t mind that very much. Sometimes, I’ve learned, you have to surrender to the sand. Fiona Cohen is the author of Curious Kids Nature Guide: Explore the Amazing Outdoors of the Pacific Northwest. She grew up in Victoria, Canada, where she developed a passion for nature, including birds, rocks, trees, and squishy things on the beach. She once sustained a black eye while fleeing from an enraged river otter. Cohen now lives in Seattle.