Did/Do you know that ?

"Do you know that..." is uncommon. "Did you know that..." is what most people would say in practice.

Of course, one is past tense and one is present tense. So you could argue that "Do you know that" is incorrect because the answer is always going to be yes. For example:

Q. Do you know that gravity is caused by the curvature of spacetime?
A. I do now because you jus

>So you could argue that "Do you know that" is incorrect because the answer
>is always going to be yes.

But we usually say "Don't you know that ...?" instead of
"Didn't you know that ..?".
Follow your line of reasoning, the answer should always be "yes"
too.

True, but "Don't you know that..." is patronizing. It more or less means "You stupid fool. Everybody knows this. I would have expected you to know this too. How come you don't?"

Anyway, my aside wasn't entirely to be taken seriously, it was just an amusing thought.
Rommie