Which side of a cat has the most fur?

“The left side”

This answer is incorrect because it specifies a direction rather than addressing the riddle's play on words about the cat's fur.

“The inside”

This answer is wrong because it takes the question literally, while the riddle is a pun suggesting that the outside of the cat is where the fur is.

“The belly”

While a cat's belly does have fur, the riddle is not asking for a specific location on the cat, but rather making a joke about the concept of 'outside' versus 'inside'.