Fruit

"We need fruits because they reduce risk of getting many chronic diseases. And, they're delicious."

- Phlegma in School of Dragons

Fruits or fruiting plants are mentioned in the How to Train Your Dragon Franchise.

Lemon
"Bork must have written this message with lemon juice ink. You see, lemon juice is more acidic than the parchment. The acid weakened the parchment where it dried. That part of the parchment turns brown before the rest of the parchment when it's exposed to heat!"

- Fishlegs Ingerman discovering a hidden message in Bork's Papers

Mango
"What a tense situation! It was nearly as bad as the one time I went into battle with nothing but my armor and a sharpened mango."

- Phlegma in The Secret of the Leviathan

Sagefruit
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