David Carter tore off a sticky note and jotted down the important points.

Things that could make Old Carter feel comfortable and happy—did it have to be something nostalgic?

Was she supposed to find a proof of Fermat’s Last Theorem to hang on the wall?

Just imagining the house covered in mathematical theorems and formulas in the future was actually kind of exciting.

After going through a pile of materials, David Carter summarized the key points. The main issue was still that Old Carter often forgot to bring his keys. As for the keys, switching to a fingerprint lock could solve the problem.

But the new neighborhood they lived in was a large, old residential area, with hundreds of buildings, each with its own small courtyard. If Old Carter went out and came back, seeing all these identical courtyards, it would be easy for him to get confused.

She’d have to do something with the door and the wall...

She stuffed the sticky note into her pocket, changed her shoes, and went out.

It was already late at night. The old neighborhood had only a few scattered streetlights. Other than the rustling of stray cats darting about, there was no other sound.

She walked through the courtyard, pushed aside the morning glory vines drooping from the doorframe, and stood outside the courtyard wall.

At her feet was a whole box of chalk.

She tilted her head back slightly, looking at the entire wall.

When she was little, their family still lived in a small house in the city center. The house leaked when it rained, but it had a small courtyard. Old Carter worked during the day and waited tables at a restaurant at night.

At nine o’clock every night, she would sit at the door, waiting for Old Carter.

On beautiful summer nights under the starlight, Old Carter would always bring back some snacks, never showing a trace of fatigue.

They would sit together under the loofah vines in the courtyard, eating snacks, and Old Carter would always try to snatch them from her, shamelessly.

While eating, Old Carter would tell all sorts of random science stories. Things like Pasteur discovering the hidden asymmetry of hydrochloric acid crystals, Franklin and the lightning rod...

Kids from the neighborhood would come listen too, because Old Carter really made stories interesting.

There was one time that was especially funny.

While Old Carter was telling a story, he was challenged by the son of a nearby professor.

A middle-schooler with a superiority complex is scary, but a little kid with one is even scarier. The boy said that everything Old Carter talked about was useless, that real science was incredibly difficult, and that Old Carter was just fooling them with silly stories.

Honestly, he wasn’t wrong...

But that boy immediately started reciting Newton’s three laws and the Pythagorean theorem and such, which was a bit intimidating.

At first, Old Carter didn’t say much, just listened with a smile. When the boy finished rattling off a string of formulas, Old Carter stood up and did something that, looking back, David Carter still thinks was extremely over-the-top.

He took the boy by the hand, walked outside, picked up half a red brick by the roadside, and, under the streetlight, wrote a formula on the courtyard wall.

E=MC2 (squared)

Old Carter: “Do you know what this is?”

“Einstein’s!” the boy said proudly. “Relativity!”

Old Carter didn’t confirm or deny it, picked up the brick again, and wrote another, even more complicated formula on the wall. “What about this one?”

The second formula started with an R, with superscripts and subscripts.

David Carter was baffled, and the boy was speechless too.

But if Old Carter stopped at a time like this, he wouldn’t be Old Carter. He kept writing, a third one, an expression with things in parentheses...

Anyway, David Carter couldn’t understand any of it.

After that, Old Carter still didn’t stop. The formulas, theorems, and equations that followed were beyond words.

Formula after formula appeared as Old Carter waved the brick, gradually emerging under the dim streetlight, spreading across the entire courtyard wall.

Finally, Old Carter tossed aside the now nearly used-up red brick and said to the boy, “Can you keep recognizing them?”

Old Carter looked very smug, full of bravado.

The boy’s face turned bright red, unable to say a word.

After a while, Old Carter slowly walked to the second equation he’d written, the one starting with R, and, unexpectedly calm, said,

“This is the field equation from general relativity.”

“This is the expression for special relativity.”

“This is the Dirac equation.”

“This is the Chern-Gauss-Bonnet theorem.”

“This is the Lorentz equation.”

“……”

“These are Maxwell’s equations.”

At last, Old Carter returned to E=MC2 (squared) and said, “And this—not relativity, but Einstein’s mass-energy equation.”

At this point, the boy finally burst into tears. The group of kids stared up at the wall full of formulas, at a loss.

Then, Old Carter crouched down, wiped the boy’s tears with his dirty hand, and asked, “Why are you crying?”

The boy, furious, could only cry and say nothing else.

Old Carter muttered to himself, “Is it because you think I, an adult, bullied you? Because you feel embarrassed in front of the other kids? Or because these formulas are just too hard and it’s overwhelming?”

Old Carter: “But just now, you did the same thing to the other kids in front of me.”

“Isn’t it annoying to show off just because you have a good memory?”

The boy cried even harder. David Carter looked around nervously, afraid the boy’s family would come out and beat up Old Carter.

But Old Carter—as if he’d ever care about that.

“There will always be someone with a better memory than you,” Old Carter said.