What is it about turning 5 that makes the digestive system so hilarious?

My grandchildren have entered a dreaded new phase of child development.

We knew it was coming. Friends whose children or grandchildren are a shade older than ours had warned us. But like so many who refuse to brace for the inevitable, I was caught unprepared. Before I could lash myself to the mast of adult reason, I was swept downriver, around the bend and right into a clump of the very stuff I'd hoped to avoid: poop jokes.

What is it about turning 5 that makes poop so hilarious? You'd think the ideal age for children to reflect upon the amusing aspects of the human digestive system would be when they're introduced to the potty. Everybody's talking about poop then: where it is, where it should be, why it should be where it should be and so on.

Then, if ever, seems like the time children would find humor in the business of waste production and disposal. But no. The toddlers I know either approach potty training with the same reverence as Phi Beta Kappa inductees, or they refuse to acknowledge the whole thing. So far, my younger grandson falls into the latter group.

One of these days, though, he will realize his older brother is an experienced potty user and rush to catch up. This is both the blessing and the curse of older siblings. Younger children—and I was one, as was my husband—always feel the pressure of being the last to know, and when what we don't know involves major life events—potty use, bike riding, licensed driving—it's agony.

Regardless of birth order, though, poop is comedic gold for 5-year-olds. Perhaps it's the last gasp of infantile behavior before they enter kindergarten and join the workaday world.

Our grandsons visited our house recently. Released from their car seats, they raced to the porch, where my husband and I were waiting. The younger boy hurled himself at us and announced in glad tones, “Poopy diaper eyes!”

“Poopy diaper eyes?” we asked, and their mother sighed.

“This comes from his brother,” she said. Ah. Not only do 5-year-olds find poop jokes hysterical, they know enough to buffer their knee-slapping humor by sending it through a younger sibling. It's as complicated as politics, this stage of early childhood development, though considerably more delightful—and I say that while not forgetting that the subject here is, as you know, poop.

In fact, parents are fairly equable when it comes to poop, probably because the first years of a baby's life involve so much of it. Even young people who start off squeamish become downright unflappable only a few weeks into parenthood. One or two diaper blowouts is all it takes to desensitize even the most fastidious among us.

Thus 5-year-olds who say “poopy diaper eyes,” then wait for the grown-ups to collapse in shock will wait a long time. This helps to shorten the poop joke phase, but it doesn't stop it altogether, because preschoolers can roll around squealing when these words are uttered, whether grown-ups notice or not. It's all part of the maturation process. Today poop jokes, tomorrow MCAT preparation.

I'm willing to wait.

Margo Bartlett and her husband have two daughters, two sons-in-law, three grandchildren and two car seats. She also writes the Just Thinking column for ThisWeek Community News. You can reach her at margo.bartlett@gmail.com.