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“Because that’s what the chloroplasts do, Cass. They take the light and they turn it into sugar. It says it right there.” Colin tapped the page with his free hand. “Photosynthesis. Photo means light. Synthesis means putting together. Light plus water plus carbon dioxide equals sugar. That’s the formula.”

“But how is light sugar?” Cassian pressed both palms flat on the table. “I’ve seen sugar, Colin. It comes in a bag. Ruari keeps three different kinds in the kitchen at Proving Ground, and one of them is called demerara. Ruari lets me eat spoonfuls of it straight from the jar while he’s baking, and it’s crunchy and golden and absolutely not made of light.”

Colin’s face collapsed into itself. “He feeds you raw sugar?”

“Spoonfuls of it, Colin. Big ones.” Cassian smiled at the memory. “And then he makes me kiss him after. He says it makes me taste so good.”

“Right,” Colin said, his mouth very thin by then. “We’ll come back to that. The plant makes the sugar from the light. It’s not — the light doesn’t become sugar, Cass, it’s the energy that powers the reaction.”

“What reaction?”

“The chemical reaction. Inside the leaf.”

“But what’s happening inside the leaf? What’s actually happening, Colin? Is it cooking? Is the leaf cooking the light into sugar, like a tiny kitchen?”

Colin closed his eyes. Tala chose this moment to yank his collar sideways with enough force to expose his collarbone, and he prised her fingers free without opening his eyes.

“It’s not cooking.”

“Then what is it? Because the workbook says the leaf absorbs photons, and I looked up photons, and photons are particles of electromagnetic radiation. Colin, I don’t know what electromagnetic radiation is, but it doesn’t sound like it’s sugar either.”

Colin started bouncing Tala on his knee. “Right,” Colin said. “The photons hit the chlorophyll, which is the green bit.”

“I know the chlorophyll. That’s why leaves are green. I got that part.”

“And the chlorophyll uses the energy from the photons to split the water molecules.”

“Into what?”

“Into — Christ, Cass, into hydrogen and oxygen, I think. The hydrogen goes off and does something with the carbon dioxide, and that’s where the sugar comes from.”

“You think?”

“Yes, Cassian, I think. I’m not a fucking botanist.” Colin’s voice came out sharper than he’d meant it to, and Cassian saw him register it and pull back. Tala babbled into the silence.

Cassian’s mobile sat face-down on the table. His solicitor was supposed to ring today with the final word on the nullity petition: whether the court had accepted the grounds or whether he’d have to sit through more hearings while strangers in wigs decided if his trauma met the threshold. The call could come at any time now, and he kept glancing at the dark screen, willing it to light up.

“But Colin, the workbook says the glucose molecule has six carbon atoms. Where do the six carbon atoms come from? You said the plant splits water, and water doesn’t have carbon in it. Water is H-two-O. I know that. So where does the carbon come from?”

Colin stared at him, and Cassian returned his look.

He and Tala played this game. They’d sit on the living room floor, nose to nose, and stare at each other until one of them laughed or looked away. Cassian won most rounds; Tala was still easily distracted by her own feet. But sometimes, on bad days, his staring made Tala cry, her face crumpling into bewildered distress.

Colin was having a bad day.

“The carbon dioxide,” Colin said, slowly. “CO2. The C is carbon.”

“Yes, but how does it get into the glucose? What’s the mechanism? Is it an enzyme? The workbook mentioned enzymes two chapters ago but didn’t say which ones do the carbon bit.”

Colin set Tala down in her bouncer, then turned his attention back to him. “Cass.”

“What?”

“I’m one chapter ahead of you in the workbook,” Colin confessed. “I’m reading one chapter ahead so I can teach you the next day, and your questions are hitting the exact fucking edge of what I actually know, and it’s making us both arseholes.” He exhaled. “I’m getting defensive since I can’t answer you, and you’re getting wound up because I’m not giving you proper answers, and we’ve been doing this dance for three weeks. I should’ve said something sooner.”

Cassian’s pen hovered over the workbook. “You don’t know how photosynthesis works?”

“I know how photosynthesis works at an Entry Level 3 standard, Cassian, which is what the course requires. You’re asking questions at a level that I genuinely can’t answer, because I left school at fifteen and became a handyman, and the last time I thought about chloroplasts was never.”