Marta’s eyes widened. "You studied chemistry?"

"Chemistry doesn’t age, child. Only the paper does."

Marta took notes in a school notebook. She didn’t need a PDF. She didn’t need a download. She needed the living, breathing, grease-stained mind of an old electrician who remembered that a book’s value isn’t in its file size, but in the questions it makes you ask.

Old Rui laughed. "In 1987, I borrowed a Química 11ª Classe from the Soviet-Cuban school library. Never returned it." He wiped his hands and disappeared into his shack. A minute later, he emerged with a battered, coffee-stained, dog-eared book. The cover was barely legible: