Let’s begin with the single representation of life that has ever made any sense.

As a young teenager, I traveled with my class on a trip to Washington, D.C. It was a three-day survey of the capital in spring, monuments and the cherry blossoms. I skinned my shin on the steep steps of the Jefferson Memorial — the shin I was always skinning. But from this journey I principally recall three events. Two are truly memorable. The third is in most ways forgettable, and occupies the highest place in my mind.

The first memory concerns a boy who saw this trip as a chance to hone his shoplifting technique; he was busted, at last, in a luggage store, sent sniffling home to New Jersey by train with a gentle chaperone. He was, he would often claim, stealing a backpack to carry everything else he had stolen. This strikes me as meaningful and also not at all.

The second memory is ritual, if emphatic, juvenilia: four of us in a cheap hotel room stayed up late one night to see Denise Richards expose her breasts in Wild Things on HBO. Next day, on the bus, each boy went out of his way to confirm he had not missed this bonanza. None could have hoped to tell you what that film is actually about.

It’s the third memory, somehow — the unremarkable one — that persists as a critical clue to existence, or the shape of the universe, or something yet more profound: I was in the gift shop of the Smithsonian National Air and Space Museum, pawing through utterly non-educational toys, wondering which I could afford. The shoplifter passed by, wearing a shoplifted T-shirt, and showed me a pencil case he’d pinched. Why did he want a pencil case? Better not to ask, I thought and still do think. He left me at last, and I came to a wide display, shelves of some bright item I’d not encountered before.

I picked one up: a cylindrical tube of soft clear plastic, heavy with blue gelatinous water, which gave it a plump form. Flecks of silver glitter sparked and tossed in tropical blueness. The real attraction was of a higher significance — a sly, topological sort. When you tried to squeeze the tube in your fist, you forced it inside out. The interior of the tube was pushed through one end to become the exterior, the exterior plunging back into the interior, et cetera, for as long the toy remained intact. The harder you tried to grip it, the less it obeyed your grasp, often squirming completely free.

Research would term this thing — until today it’s been the thing — a “Water Wiggler” or “Water Snake” or “Squeeze Water.” Common enough, they sometimes contain fake fish or other tidbits. To my class, anyway, they were a novelty, and soon most of us were playing with one, there in the gift shop, silent a while, each exploring the exact nature of human fine motor skills, how fingers could manipulate this wholly foreign object.

The kid who bought one dropped it in the dirt outside, which dusted the sweaty surface and ruined forever the odd sensation of touching this riddle of a toy. Just as well. Experience was confined to itself. Couldn’t have happened in any other place, at any other moment, for any other reason. The thing, I later intuited, was life itself: if you tried to hold on, you found yourself holding something entirely changed. Or nothing.

Recently I found myself dissatisfied by the dictionary. It was over the word “afterthought,” casually denigrated as a realization — a clarification — that comes too late, does too little. L’esprit de l’escalier, to put a finer point on it: a thought you should have thought of sooner. Can we think of thoughts, or can we only think them?

It’s confusing, and the prescriptivists aren’t helping. I carry my own definition.

For me the afterthought, if it even exists, and it probably doesn’t, is this: the thought you were not ready to have. A latent cognition preserved until you can take it apart and put it back together. “Of course,” I tell myself, again remembering the Water Wiggler. “The Water Wiggler explains my life; I just hadn’t lived enough to see that.”

We strangely suffer the instinct to hold on regardless, to keep the shape of this pocket of space-time pure in the neural thicket of nostalgia. In case we wish to read it later. Maybe it’s not pure. Maybe it wasn’t the Smithsonian. A few details are beyond editing: the shoplifter’s yellow-and-red pencil case; the squishy toy in my hands; the toy when it had fallen in the dirt and no one wanted it anymore. Who would want such a thing.

My wife has her own memory like this. She was fond of puzzle books but happened once to glimpse a page from an answer key. The answer she saw was “camel,” alongside an illustration. She decided to save this puzzle until she’d forgotten about the camel, but naturally this was impossible — twenty years on, she remembers a camel. The answer is camel. In the back of the book of life are some answers, and one of them is: camel.