For more than 20 years, I’ve repeated a story that ends in an enigma. Expecting a punch line, people who listen to it get a puzzle. Here it is:

At the start of the school year, a dozen Apple Valley first-graders—half of them boys, half of them girls, one of them my daughter, Amanda—were assigned to an unsupervised bus stop a few houses down the street from us. From the first day of school, while every one of the girls stood in an orderly line, the boys roughhoused. They ran away with each other’s caps. They gathered piles of leaves and threw them into the street. They stomped in puddles, for no apparent reason but to be annoying.

 Winter arrived. The boys threw snowballs—at a stop sign, the nearest house, and each other. They marched over front yards to make footprints in new-fallen snow, kicked plowed snow onto newly shoveled sidewalks, and jumped into drifts from a section of decorative fencing. The girls stood quietly in line, ignoring the boys.

Heisenberg discovered that the movement of electrons is altered under the light of the microscope needed to observe them.

Then in late January, the bus stop was divided into two—and coincidentally, all of the boys were assigned to one, the girls to the other. The boys did not seem to notice, and their behavior did not noticeably change. But the girls’ behavior changed instantly and dramatically. They began immediately to behave like the boys.

 They threw snowballs. They battered down a snowdrift. One shook my mailbox. And there was Amanda, tromping in her baby-blue boots across our snow-covered lawn to gleefully heave the largest block of snow she could possibly carry onto the driveway that I had cleared the previous evening.

It is possible that there is a feminist explanation for the two sets of behaviors, or a theory of developmental psychology. If so, I can’t find one.