I know it

[ref]

On Sat, 9 Jun 2001, Benjamin Barrett wrote:

> I've been wondering for some time if there's been a change in the
> way questions are answered.
>
> A: Did you know that there is a leap day in the year 2000?
> B: Yes, I know it/that.
>
> To me, only "that" is correct in this context, but I hear older
> speakers use "it."
>
> To me, "I know it" has a slightly different intonation and means "I
> agree with you" or "I know what you're talking about."
>
> Has anyone else noticed this?

I remember when I was a child noticing the "I know it" construction in
at least one book and being very puzzled by it (children's fiction -- I'm
fairly certain it was one of the Judy Blume books, possibly _Tales
of a Fourth Grade Nothing_).  I was puzzled because, for me, it would have
to be "I know that", and it was difficult for me to imagine someone saying
"I know it" instead.

I now recognize the distinction you give, though I'm not sure I'd use
that modern "I know it" myself; this is not, however, how "I
know it" was used in the Blume book or whatever it was.