[Rhythm of War] The Boot, the Rat, and the Spren by Pam Hage

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Relevent book quote (lifted from deviantart):

“I don’t think he ever gets excited. Not even when I tell him I have a fun surprise for him.”

“Your surprises,” Kaladin said, “are never fun.”

“I put a rat in his boot,” Syl whispered. “It took me forever. I can’t lift something so heavy, so I had to lead it with food.”

“Why in the Stormfather’s name,” Lirin said, “would you put a rat in his boot?”

“Because it fit so well!” Syl said. “How can you not see how great the idea was?”

• Rhythm of War, chapter 18