In the annals of parenting literature, I cannot find one book that specifies the age at which children understand how knock-knock jokes work.

“Knock-knock!” says my 6-year-old daughter, Fern.

With gathering fear, I answer, “Who’s there?”

“Chicken!” she says.

“Chicken who?”

“Chicken strips!”

I sigh and try to explain. “No, it’s a play on words, like ‘Boo who?’ is funny because it sounds like crying...”

But it’s useless. Fern laughs maniacally and runs off to share her joke with her mom, sister and dog.

I console myself first that the punch line wasn’t “chicken butt” (as usual), and second, that at least there’s something left I can teach her.

Full-time parenting differs from other jobs in one crucial way: you never get to be an expert. Every few months, just as you’ve mastered one developmental phase, your kids change and your old skills are washed away, obsolete.

Fern and her little sister Claudia love crafts more than anything, but they mostly outgrow every project as soon I teach them. Finally, I found an activity I thought they’d enjoy for years to come: a “grass buddy.”  You fill a nylon stocking with soil and seeds, glue on some googly eyes, add water and watch your new living head grow “hair” over the next few months.

Every year, the girls looked forward to the day we brought the grass buddy activity to our cooperative preschool. But it seems that even the best projects have a limited lifespan. Now that Claudia is 4, she took no time in unenthusiastically filling her buddy and then dashed off to build castles. She’s an ambitious kid and scooping dirt into a sock no longer inspired her.

But even before I started perusing craft magazines for a new project, Fern herself showed me what she needed to learn – by making me a picture.

A gory picture, in fact. In crayon and pencil, the drawing depicted Fern – who’s just a first grader – knocking a friend down to the ground, then handing her a Band Aid for her bloody knee. For a caption she wrote, “Good friends can say sorry when you hurt them.”

“Yikes!” I said, almost out loud. I took a breath and asked what had happened. It turns out Fern is in a predicament far more delicate than mere physical confrontation: tension between friends.

Here’s how Fern told the tale: she has an old friend, but the old friend made a new friend, and mostly they’re all friends, except when they’re not, and then Fern feels left out, and then she gets bossy and that makes it all worse.

No matter how well I teach about seeds and soil, I am oblivious as to how little girls grow into lifelong friends. Does minor conflict act as fertilizer or poison the water?

And so my job has evolved from teaching popsicle-stick construction into counseling about interpersonal relations, and I’m no expert. But I suppose the first step is obvious: I need to find time to talk with Fern about companionship and empathy and even loss. For a while, our conversations will revolve less around glue sticks and more around being a good friend.

And maybe, as long as we’re talking, I’ll squeeze in a decent explanation of knock-knock jokes.

Graham Charles blogs at Doodaddy.net.