It was one of those days that I could have missed. So easily, I could have missed the first time William put something in a bucket (to this point he's only taken things out). I could have missed Penny's applause, "Good job, William!" I could have missed him running into the water on his short little legs, yelping with delight at the cold waves against his ankles. I could have missed Penny learning to get her summer feet and walk across the shells.

Just an hour earlier, 4:00 p.m., Penny was asleep. The plan had been to take our kids to the beach for some time on the sandbar. The tide would be low at 4:30, and I had envisioned the two of them running on the dark brown sand, splashing in pools of water, peering at barnacles on the underside of rocks.

But 8 other family members were coming for dinner, and I was in the middle of editing an essay, and I hadn't taken a run, and Penny was still asleep... Maybe, I thought, we should just wait until tomorrow. Yes, that makes more sense. I ran upstairs to take a shower and get to work on dinner.

When I emerged from the shower, there they were. Penny in her lime green bathing suit, William in an orange baseball cap, Peter with a towel slung over his shoulder. 4:45. They didn't know I had changed the plan. Off we go.

We arrived, and there was no sandbar, and Penny took one step on the beach littered with shells and said, "I don't like it." William keeled over and hit his lip on a bucket. Screams. Blood. And dinnertime fast approaching. But then, as Peter consoled William, Penny and I started picking up seashells. A small one–white, flat, like a shaving from a bar of Ivory soap. A translucent yellow. A thick and creamy one with deep purple. We started a collection. William soon joined us, running uphill to dig through the pile of boat shells at the top of the beach.

It wasn't just moments of wonder with my children that I could have missed yesterday afternoon. As I sat next to Penny, my mind wandered back to when I was a little girl, taking the time to walk along the beach, picking up shells and sea glass. We had made lamps and decorated mirrors and run home with handfuls of the ocean's treasure as offerings. Until yesterday, I hadn't noticed how much the beach had changed. The shells were different. The translucent ones, the ones we always called toenail shells, were bigger, flatter, more beautiful than I remembered. The sandbar had moved. I didn't find any sea glass. The flat, white shells were new to me.

So I came home sandy, a little bit wet from salt water lapping at my feet. My extended family arrived and dinner wasn't ready. The essay wasn't finished and I never got that run. But I had an hour with my husband and our children, an hour to watch them grow up one step more. An hour to realize that the world had changed beneath my feet. I could have missed it.