Explanatory form のです used in two consecutive sentences


I am learning the explanatory form のです. My book gives the example:

> ケーキはもうないのです。犬がぜんぶ食べたのです。
>
> There is no more cake. That's because the dog ate everything.

I don’t understand why they use the explanatory form in both sentences, I
would think it is used only in the last one. All the other examples I have
found work that way.

Can anyone confirm that this example is good, and explain why? Thank you!




You are right, the first sentence looks fairly unnatural to me if this is
everything in the conversation. You have to say ケーキがなくなってしまいました or something
if this is the first sentence in a conversation.

However, this may be valid if there is enough context before this, for
example:

> A: 昨日買ったケーキを持ってきてくれ。  
>  B: 申し訳ありません、ケーキはもうないのです。犬がぜんぶ食べたのです。

Now ケーキはもうない works as the reason for saying "I'm sorry", so the use of
explanatory-の in that position is also natural. (As an aside, んです is much more
common than のです in speech, even in formal settings.)

