In Sweden, it's already Saturday ;)
I'd like to take this opportunity to point out that in English, the natural answer to this question would be It is, but in Swedish, we'd say Det är det, and both det are necessary. The course doesn't really do a good job at teaching this because a sentence like this doesn't work well on its own, and dialogue sentences are so glitchy that we're not supposed to use them. We still have a couple, to illustrate the use of jo, and in those you can see the same phenomenon: we say Jo, det gör jag where in English you'd just say Yes, I do.
Similar examples: Kan du läsa? Ja det kan jag 'Can you read? Yes I can'
Vill du gå hem? Nej det vill jag inte 'Do you want to go home? No I don't (want to).'
Är du trött? Nej det är jag inte 'Are you tired? No, I'm not'
The extra det is a placeholder which is there for purely grammatical reasons, but in most cases you can't take it out.