"What the matter?"

"I just have a feeling."

"He's lying there peacefully, Mummy, don't bother him."

"No," I said triumphantly, reaching down between Billy's guilty paws and removing an object. "He's not just lying there. He's secretly eating ... this!"

I raised the thing in the air and laughed. Billy had somehow got hold of the skin of half of an avocado, a little dark green cup that he had covertly been nuzzling.

He knew he wasn't supposed to have it. He had been lying still, trying not to draw attention to himself. The pack imperative meant he felt he really must stay close to us, but perhaps if he just could stretch out, prone, with his head facing away from the all-seeing alpha, he might get away with exploring this strange, interesting and possibly tasty something.

Ah, but he'd been busted! How could that happen? How did she know?

The same questions had run earlier in the week through the mind of Billy's, er, older brother. The boy had come home after school as innocently fresh-faced as a juvenile saint. Yet, within seconds, his mother intuited that something was up. All was not as it was supposed to have been; he had not been where he was supposed to have been.

After a short series of questions and answers, the truth emerged, and in a prefiguring of the scene with the avocado skin, the boy's mother plucked the iPod from his paws and confiscated it.

He'd been busted. But how did she know? I wish I knew the answer.

It's feels so mysterious, this process by which cold understanding sometimes passes into the mind. It's as though hundreds of tiny clues are whirling about invisibly, like dust in the air. And then, just as a shaft of sunlight may suddenly illuminate the dust, knowledge flashes into your mind. The clues light up, and they form an arrow that points to the culprit.

The funny thing, when it happens, is that you may know there's a culprit before you even know what crime has been committed.

An old friend once told me that his Irish mother could tell instantly when any one of her six children was misbehaving. She didn't even have to turn her head.

"Sometimes she'd punish us right that minute," he told me. "We'd shout: 'Ma! You weren't even looking! How d'you know it was me?' But she knew. She always knew."

I envy that degree of infallibility, because it might have prevented last night's disaster.

Clues, it seems, do not simply float around like dust motes. It turns out that a clue can manifest as a fugitive gnawing noise that's just barely perceptible amid the general hubbub of a busy household on a school night.

At some level, I registered the crunching sound, but it did not signify. I was also vaguely aware that the dog had placed himself at an unusually far remove from the rest of the pack. When he eventually loped over to join us, the arrow of blame was formed not of motes, but of great, inky blotches.

"Oh no, look at Billy's paws!"

"Look at his muzzle!"

"Help!" I shrieked, "The dog ate a pen!"

The signs had all been there. Unfortunately, on this occasion I'd been totally ... clueless.

Meghan Cox Gurdon's column appears on Sunday and Thursday. She can be contacted at mgurdon@washingtonexaminer.com.