“Let’s just say,” whispered Leo, “we found a very useful PDF.”

Clara smiled. “It’s full of past exams. Look: here are diagrams of food chains, fill-in-the-blank questions about photosynthesis, and even a section on classifying vertebrates.”

Clara breezed through the section on classifying animals, and Mateo — the one who had been most afraid — correctly wrote that fish breathe through gills, not lungs.

When the exams were returned three days later, all three friends had scored an "Excellent."

Leo thought hard. “They… make their own food? Like plants?”

“Vertebrate! It has a backbone!” shouted Leo.

For three afternoons, they worked through the PDF. They learned that herbivores eat only plants, carnivores eat meat, and omnivores eat both. They practiced labeling the parts of a flower — root, stem, leaf, and the tricky one, the pistil.

Next, Clara pointed to a question from the PDF: “What do producers do?”