The House of the Scorpion

From the text:

“Mamá,” he said without thinking, “tell me again about the kids in the Big House.”
“Don’t call me ‘Mamÿ,’” snapped Celia.
“Sorry,” said Matt. The word had slipped out. Celia had told him long ago that she wasn’t his real mother. The children on TV had mamÿs, though, and Matt had fallen into the habit of thinking of Celia that way.
“I love you more than anything in the world,” the woman said quickly. “Never forget that. But you were only loaned to me, mi vida.”