sequence of tenses

[greegorush]

So - why are we far more likely to say:
"I didn't know that water boils at 100º Celsius.
and even more (exaggerating for emphasis):
"I didn't know there are stars in the sky."
Why Past tense in the examples above, yet Present tense in these sentences?

Firstly thanks a lot!
Now I'll try to answer your question:
"I didn't know that water boils at 100º Celsius." I know that water boils but did not know at what temperature.
"I didn't know there are stars in the sky." Here I'm not sure that I'm right...More likely that - I see something in the sky but did not know these are stars.

Let me know please if something wrong or there's a better explanation there.