"If you don't wanna be a cat, what about a dog?" suggests Radar. With his attention divided between Fever and his own work, he misses the part where she didn't say most of that out loud. "Sometimes they're easier to catch than cats. Or maybe somethin' else kinda slow moving?"

Maybe he's being too literal about it, but if they're both thinking about the spell as something you catch -- well, it makes sense to him.