A little child’s mind is like a sponge, while an aging person’s is more like a sieve.

Everyone knows that. It becomes more apparent to me as I grow older and watch people I know well struggle to remember things they’ve known all their lives. It’s especially heartbreaking when you know it isn’t simply a momentary lapse but rather a sign of a progressive, irreversible collapse of memory. I doubt there are many families that haven’t watched that happen to a loved one.

At the same time, Nancy and I have spent our recent days caring for a great-granddaughter who is now eight and one-half months old. Unless she’s dreadfully sleepy or ravenously hungry, every single thing in her world captures her complete attention. Lately she’s tried to tell us important things about those things she encounters. It’s an amazing thing to witness. Every new day brings her greater recognition of her world and seems to make her even more determined to tell us what she’s discovering.

When we first began caring for the child, she mostly ate and slept. She cooed now and then, and she cried when she needed something, although it was often a big guessing game to figure out what it was she needed. Sometimes she would cry after a nap, a bottle and a change of diapers, and I’d say, “Use your words, Cedar.’’ That seemed to make her complain even more emphatically, but it was kind of a funny thing to say. I thought so, anyway.

Well, now she’s starting to use her words. Recently she’s been actually saying “Dada,’’ which made everyone in her world swoon. Just the other morning when her father left her in our care, I swear she said, “Bye bye, Da.’’ If you doubt me, just ask Grandma Nancy.

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Of course, Grandma Nancy is pretty much convinced the child has been recognizing and naming objects and people for ever so long. She just doesn’t use words and phrases adults can recognize. And I have to say, the kid carries on some pretty intense conversations with her books, rabbit, singing elephant and her reflection in the mirror. She’s recognizing things in her world and trying to put names to them. She’s soaking up knowledge just like a big old sponge.

That’s how kids are. I recall being in second-year German in college. “Com and Con,’’ we called it. Composition and Conversation. Writing and talking. The instructor sometimes brought 4- and 5-year-olds from the nearby day care into our classroom to talk German to us, demonstrating how easy a language it was to pick up. And it was, for those little kids. They sopped up every syllable. By contrast, every day in class I felt I was flailing to learn each new word and pronunciation. And I was just 20.

These days, 57 years later, I’m aware that I no longer learn quickly. Too often my concentration slips as I read a book or magazine, even if the subject matter interests me. Some of that may be the online age of short attention spans, but I think some of it is simply growing old. I sometimes laugh and say my brain is full and can hold no new information. But I’m also aware that, even though I have a reputation for remembering the past, some information is leaking away through the small holes in the sieve.

A newspaper publisher I admired told me at age 90, “I can remember things from the past like yesterday, but yesterday is a blur.’’

It’s difficult to know when that kind of memory lapse is just the “Where’d I leave the keys’’ aspect of aging and when it becomes an actual, progressive loss of memory. I’ve had close friends who lived their final years with Alzheimer’s disease, and it tore my heart apart to see them slowly disappear. Forget the keys? How about the family? How about no longer recognizing any of the things in the world, those things that our little girl is so quickly, eagerly learning?

It’s amazing to see a child absorb knowledge, terribly hard to watch the process in reverse.