Tag Questions

Since we use these indefinite pronouns this way:

Everyone thinks they have the answer.
Has anybody brought their camera?
No one could have blamed themselves for that.

We use all of them with they, in tag questions:

Somebody told you the news, didn't they?
Everybody is here, aren't they?

But remember that nobody is already negative so the tag must be in the affirmative form:

Nobody knows, do they?