The fourth grader gives herself a point for spotting the zebra on her zoo card. It is Equus grevyi, Grevy's zebra, whose stripes terminate in an all-white belly. Grant's zebras are smaller and have stripes that go all the way around.
It is the fourth grader's game, played with the fourth grader's cards, by the fourth grader's rules, which she is making up as she goes along.
But we are taking it very seriously, because otherwise we would just be enjoying a day at the zoo like normal people.
First, we stand at the sign in front of the zebra enclosure for a really long time. The zoo wants us to guess. "Which type of zebra do you see?" But we're not going to start awarding points on the basis of a guess. That would be anarchy.
"Grant's zebras," the seventh grader announces to his sister. "Zero points for you."
She disagrees. "No," she says. "Those are Grevy's."
"They are not," says the seventh grader, in a voice you might use to point out a zoo animal to a preschooler. "Look at them. They do not have white bellies."
"It's hard to tell," I say casually, as if there is nothing riding on this. But I know: our entire day at the zoo is riding on this. Other families begin to move away.
He reads the description of Grant's zebras from the sign, in a too-loud voice that suggests his near-constant frustration at being the only spark of intelligence in a family of idiots. "They are Grant's," he declares. End of story.
The fourth grader pretends not to hear. "One point for me," she sings.
I have been given the card for Hyla gratiosa, the barking tree frog, and have no chance in this game. But rules are rules, even if the deck has been stacked and even if no one knows exactly what the rules are.
"Let's read the sign again," I suggest. "Maybe there is another sign. Let's ask somebody."
But they are too engrossed in a discussion of mammalian classification to pay any attention to me. The discussion goes like this:
I try de-escalation. "What do they mean by 'white' anyway? Their bellies are mostly white, but you can't really tell where the stripes end."
Which turns out to contain a single Grevy's zebra, much larger than the previous zebras, with an obviously white belly and - perhaps most significantly - a sign in front that reads: "Grevy's zebra."
They are the largest type of zebra, weighing up to 990 pounds. They don't hang out in herds and unlike other members of the horse family, our zoo card tells us, "the only strong social bond is between a mother and her offspring." But how do they get along? I wonder.
The boy we call Boy, Esquire is never going to let this go. He is only briefly distracted by the possibility of a new argument called addax or ibex? He will still be crowing about zebras on the drive home, "Just say it," he will repeat more than once. "Who was right and who was wrong?"
The girl doesn't care. Either way she wins.