Coleman siblings Ray and Zia chase each other on a recent trip to the Nelson Atkins Museum of Art in Kansas City, Mo.

A big brother wonders: What are little sisters for? I’ve watched it almost three years now, as our son Ray trades barbs and blows with our daughter Zia, who has never been shy or gentle while jostling for slops from the household trough of love. Little sisters grab all your stuff, bust up your games, scrunch up your artwork. Once, it was always your turn. Now, the baby born to press your buttons gets one, too.

But a big brother moves so fast mosquitoes hardly bite him, let alone his little sister. My son uses this strategy to best effect at the table, where he bolts for his bread and flees before his sister has begun. She meanders through her meals at a Continental pace. But then again, she is buckled in.

Daddy Rules columnist Dan Coleman and his children, Ray and Zia

So Ray streaks, comet-like, past Zia in her high chair, and I observe these heavenly bodies from behind the dome of my own observatory; it’s no Palomar, but I milk my morning paper — age-old refuge of dads the world over — for all the distance it offers. From here I see my kids defy Newton’s laws as well as my own, when the unquantifiable force of sheer curiosity halts Ray, an object in motion if ever there was, in mid-orbit. A big brother is always on his way somewhere, but that place rarely holds more interest than just what exactly his little sister is up to.

One day, Zia had finished her breakfast and was playing with the small spray bottle full of water I use to detangle her prodigious bedhead, having been advised by Grandma that my parenting skills would largely be judged on how tangled her hair appeared in public. In the process of Zia’s investigations, she turned the spray bottle on herself and received an unexpected, point-blank misting, after the cold shock of which she realized what had occurred, burst out laughing, and did it again.

Ray happened by at just this moment and found the whole process so utterly hilarious he had to try. Zia, more than happy to oblige, handed him the bottle, and soon followed the all-too familiar chorus of a big brother and little sister up to something together: “Again, again, again!” In our house laughter usually accompanies this refrain, which, often as not, becomes “stop, stop, stop!” before it ends in tears, but always means something interesting is afoot. I can’t begin to describe my own rabid interest in the spectacle of my kids taking turns shooting themselves in the face with cold water. This was like a dunk tank I didn’t even have to buy a ticket for, and my ball hit the bull’s-eye every time. “Need a refill? Don’t move. I’ll get it.”

Another day, Zia sat unsuccessfully stacking frozen blueberries, which had lost all culinary interest but now stood in for construction materials otherwise scarce at the dinner table. “This is my sandcastle,” she kept saying. My wife Audrey and I, busy building our own castles in the air, provided enough oohs and aahs of inattentive affirmation to keep her going until Ray could resist no more. Drawn from whatever far-flung corner of the universe, he found her Taj Mahal to be nothing more than a plate of squishy purple blobs.

“That’s not a sandcastle,” he informed his little sister.

Someone had to tell her, I guess. My wife and I snapped to full attention, aware of how quickly such an interaction could become a supernova. All was quiet. Claude the dog wisely heaved himself up and limped down the hall as fast as his ancient haunches allowed. Out of the corner of my eye, I thought I saw a tumbleweed roll by in the living room.

“Yes it is,” Zia replied, a retort so absurd and cheerfully delivered that all anyone could do, including her, was laugh. For once the instinct to oppose her big brother, a limitless fund of currency that every little sister is heir to, paid dividends in mirth instead of rage.

But only Zia was laughing a few weeks ago on her first day of preschool. Of course we had been most worried about her, but when we pulled out of the driveway, honked twice, and left her chair empty for the morning, Ray took it hardest. Surprising us all, bolstered though he was between his mom and grandma, he refused to wave goodbye, then fell apart on the stoop as we drove away.

Perhaps now, having pondered the question these past few years, he shared my own conclusion. And past the time his mom and I are dead and gone, I pray the answer sticks.

What’s a little sister for, Ray? You. Just as you, big brother, are for her.

— Dan Coleman is secretary on the board of Dads of Douglas County. He is a part-time stay at home dad with a 2-year-old and a 4-year-old, but in his other life he is a librarian at the Lawrence Public Library, where he selects children’s and parenting books for the Children’s Room. He can be reached at danielfcoleman@yahoo.com.