I never expected my sign to come in the form of a plastic bag.

As a parent, you’re told over and over how fast it all goes, to cherish these times because they’re gone in a blink. You see the gradual changes in your kids as they move through milestones. One day, they can hold their own spoon. They begin stringing words into sentences. Their ages are counted in years and no longer months.

You watch these things happen every day, but I didn’t realize some transitions would come in tangible ways, like a grocery bag filled with wet swim trunks.

My son recently had a splash day at his preschool. The night before, I tucked $2 for the ice cream truck in his backpack, placed his change of clothes in it, and stashed an empty grocery bag on top. I showed him where each thing was so he could tell his teacher and let him know the grocery bag was for his wet clothes. He nodded along, and then I zipped his backpack closed.

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The next morning I dressed him in his swim trunks and reminded him he had dry clothes in his backpack before walking him into his class.

I hugged him goodbye and watched him run over to his friends, giddy with excitement.

Hours later, he breezed through our front door in his spare clothes and happily explained to me all the games they had played in the mini pools, the colors of the popsicle he had picked from the ice cream truck, and who he got to sit by at lunch.

When he was finished, I asked, “Which teacher helped you get changed today?” And then I waited to hear who I should thank for making sure my boy had gotten his shirt on the right way.

“No one, Mama. I did it myself! And look!” He pulled me up from his bedroom floor and over to his backpack. He peeled open the zipper and grabbed the grocery bag out from inside. “I put my wet clothes in here just like you said, and I even tied it shut.”

I studied his makeshift knot, a heap of twists and loops his little fingers had worked so hard to secure, and I was overcome with emotion.

He took off to play, but I couldn’t stop staring at the bag.

I hadn’t realized we were already at this level of independence, but this bag was a clear indication. He had been dressing himself at home for a while, but shirts still ended up backward and an occasional sock would be inside out. My husband and I were always there to snap tough buttons or straighten out waistbands.

But in a single afternoon, in the comfort of his classroom, he had grown, and the proof of that growth was sitting on my couch in a wet bundle inside an old Target bag. It made my heart hurt in all the best ways.

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As my kids get older, I understand how rare it is to hold your child’s milestone in your hands.

Growth is one of those things that slips through your fingers like time—you don’t always see it until you look back.

So we take pictures. We record videos. We mark up the walls with pencil lines and measurements, hoping these snapshots and markings will make their changes something we can hold onto, something more concrete.

But every so often, we get lucky. Every so often, growth is tangible, like when your boy brings home a knotted-up grocery bag full of wet clothes, and that bag lets you feel him in two places at once—little and a little bigger.

When that happens, you don’t have to watch their milestones slip through your fingers. Instead, you get to do what I did—you get to run your thumbs over the twisted plastic loops, soak in the smell of summer pool water, and acknowledge that your child’s growth can make even an old grocery bag feel like magic.