MY DAUGHTER HADN'T LAUGHED SINCE HER MOTHER DIED — THEN I FOUND HER ON A CURB BESIDE A HOMELESS GIRL

The room on the third floor of the Cole & Sterling building on Franklin Street had not been occupied since nineteen-eighty-four.

It had high, arched windows that looked out over the elevated train tracks, floors of narrow pine boards that had been oiled until they were the color of dark molasses, and an antique cast-iron radiator that hissed with a gentle, reassuring warmth whenever the wind came off the river.

By six o'clock that evening, Marcus and Calder had cleared out the old drafting tables and the rolls of yellowed linen blueprints.

A twin bed with clean linen sheets stood against the brick wall. A small oak desk, a gooseneck brass lamp, and a brand-new wool sweater—navy blue, identical to the one Sophie wore under her school uniform—rested on the mattress.

Maren stood in the center of the room, her duct-taped backpack resting on the floor between her boots.

She had showered in the executive washroom on the sixth floor. Her hair was still damp, combed back from her face with a wide-toothed comb Calder had bought at the corner drugstore. Without the layer of soot and street dust, she looked younger—almost fragile—the high, sharp line of her cheekbones standing out like the ribs of a racing hull.

The door to the hallway opened.

Sophie stood in the threshold.

She was wearing her flannel pajamas and her wool socks, her small hands holding a wooden bowl filled with sliced apples and sharp cheddar cheese that Mrs. Gable had prepared before leaving for the evening.

Neither of them spoke for nearly thirty seconds.

The room filled with the rhythmic, mechanical clatter of an eight-car Brown Line train curving around the bend at Merchandise Mart, its windows flashing with yellow light against the high plaster ceiling.

Sophie walked across the pine floorboards. She did not look at Maren’s face; she looked at Maren’s feet.

Maren had taken off the split canvas sneakers. She was wearing a pair of thick, gray wool socks that Calder had left on the desk.

Sophie set the wooden bowl on the corner of the oak table.

“The third problem on page eighty-four,” Sophie said, her voice small, clear, and perfectly steady. “The one with the two trains leaving the station at different angles.”

Maren turned her head, a faint, dry twitch appearing at the corner of her lips.

“What about them?”

“It’s not a train problem,” Sophie said. “It’s a triangle problem. But they didn't give the angle between the tracks.”

Maren sat down in the wooden desk chair. She reached into the bowl, picked up a slice of apple, and bit into it with a crisp, sharp crunch that sounded like wood splitting in cold air.

“They didn't give you the angle,” Maren said, chewing slowly, “because the tracks are running along the lines of longitude. If they’re running along lines of longitude, what happens to the distance between them as they go north?”

Sophie’s eyes went wide. Her mouth opened.

“They meet,” Sophie whispered. “At the pole.”

“At the pole,” Maren agreed. “The triangle isn't flat. It’s sitting on a ball. You’re doing spherical trigonometry on a flat piece of paper because your teacher thinks the earth is a table.”

Sophie let out a breath—a small, soft puff of air that was half-giggle, half-gasp.

She pulled the second chair out from beneath the desk, climbed up onto the seat with her feet tucked under her thighs, and laid her spiral notebook open beneath the brass lamp.

I stood in the darkened hallway, outside the pool of amber light that spilled through the open door, watching the two of them through the six-inch gap in the frame.

Maren did not take Sophie’s pencil. She did not lean over the child’s shoulder to guide her hand.

She sat with her arms crossed inside the sleeves of her navy sweater, her chin resting on her chest, her pale green eyes tracking the movement of the yellow pencil across the white paper with the cold, immovable patience of a surveyor watching a plumb line settle.

“Draw the sphere first,” Maren ordered softly. “Don't draw the trains. The trains don't matter. The trains are just iron moving through the cold. Draw the curve that holds them.”

Sophie drew.