Let's try to determine our models from the preceding specifications. Post is obviously a model, as it is the main feature of our blog (we display posts). Each post is created and updated by an authenticated user, meaning that users have to be saved into the database; therefore, we also have a User model. There can be posts without comments, and categories without any posts, meaning they belong to distinct models; therefore, there is also a Category and a Comment model.
That sums up to four models: