My 5-year-old grandson was building a wheeled Lego something, and I was his parts department.

"I'm looking for a blue rectangle," he'd say, and I'd go to work, scrabbling through the yellow bin of Lego pieces and holding up blue rectangles for his inspection.

"No," he'd say to this blue rectangle and that one. Eventually, I'd find the correct piece, and wait for his next request.

"Hey, here's a joke," I told him. This was a good joke. He was going to love it.

"Knock, knock," I said.

"Who's there?" he responded, still working away.

"Interrupting Cow," I said.

"Inter -- Oh, I already know that one," he said, delighted to be ahead of me. Nothing gives 5-year-olds -- or 4-year-olds or 3-three-olds, for that matter -- more pleasure than knowing more than their grandparents.

Several years ago, my husband asked my granddaughter -- now teetering on the cusp of 5 herself -- where her parents kept the dog's leash, and she led us to the leash like Merlin leading young Arthur to Excalibur. Her pride in knowing something we didn't know positively blazed, which is why we remember the incident and also where the dog's leash is kept. (In the kitchen alcove, near the recycling bin.)

Now, of course, we make it a point to ask the grandchildren which cup is theirs, where in their house we might find a Band-Aid and how to open their folding kitchen stepladder, because they're always eager to share their domestic knowledge. In fact, I'm thinking of writing a pamphlet for the State Department. It will be called "Ask, Don't Tell," and it will explain how to draw information from foreign adversaries without threatening them with annihilation or other violence.

Instead of barking, "Show us your nuclear silos right now!" a diplomat could say, "Do you know where you keep your nuclear silos?" and the adversaries, flushed with importance, would march right to them and point.

Of course, there's always a downside -- in this case, that the United Nations constantly would be bringing a book to the diplomat and asking him to read it. Not that reading aloud is a downside; it's reading "Baby Monkey, Private Eye" in all six official U.N. languages five times each that begins to wear on a person.

I didn't begin to tell the joke to my grandson so he could have the pleasure of cutting me off. I thought I was about to give him hours of joke-telling pleasure.

My husband told this same boy a joke awhile back and encouraged him to repeat it to his parents. The joke was received with such enthusiasm that he immediately started cranking out original gags: "Why did the elephant walk down the street? Because he wanted to!" "Why was the cat furry? Because he was a cat!" Each punchline triggered uproarious laughter, partly because you'd be a pretty crabby person not to laugh with a chortling child, and partly because the child's near-hysteria at his own wit was so contagious.

Recently, however, our two older grandchildren entered the next stage of life, one at which they both understand and recite actual jokes. The knock-knock joke I'd tried to tell our grandson was one he'd have appreciated, had somebody not beat me to it, unlike a joke I came across as a 9-year-old reader of Highlights magazine. Here's the Highlights joke:

Waiter: Why are you pulling up the carpet?

Restaurant customer: I wanted to see the floor show.

I was nonplussed by this joke. I had no idea what a floor show was, and even today I'm surprised the then-editors of the magazine assumed children would. How many children dine at restaurants with floor shows, after all?

Instead, Highlights should have printed this hilarious exchange:

Knock, knock.

Who's there?

Interrupting Cow.

Interrupt --

MOO!

Oh, you've heard it? A boy with Legos and impossibly long eyelashes got in ahead of me, I suppose. I wouldn't be a bit surprised.

Write to Margo Bartlett at margo.bartlett@gmail.com.