I agree with the above answers, but I can't help but feel we're cheating.

For one thing, we never explicitly gave a functor from \$$\mathbf{Set}\$$ to \$$\mathbb{N}\$$, even though \$$\mathbb{N}\$$ is certainly a category.

I think the construction of such a functor would be a good exercise, given it also reinforces the notion of composition of functors.