Now I know someone's already asked "Which is correct? “Did you know?” or “Do you know?”", but the question was unclear. Mine could be more specific.
Should I use "did you know" or "do you know" to introduce a fact?
I've only seen "did you know" in action. My logical deduction is that before the "question" (which is not much of a question because you're not asking for an answer), you wouldn't have been sure whether the listener'd known about what you're about to say or not. But by the time you say the "question" and introduce the fact(oid), you would know for sure they do. Why? Because you've just told them! It's like when saying "Did you know strawberries aren't berries, but bananas are?", I didn't know if you'd already known about that factoid prior to my "question", but at the time I say it onward, I know you know it because I've just told you. Therefore, it'd be reasonable to use the past tense with "did". It'd be pretty much the same with "What if I told you…"
(And I just realized I made it pretty confusing by using a lot of "know's")
However, a public speaker (who I'm supposing doesn't have a very good grasp of English because she thought a "kickass person" meant someone who encourages people to do things by kicking in their asses) told me that when you said "did you know", you implied that the listener'd already known what you were about to say, which is completely opposite to my reasoning. If I'd imply such a thing, I'd use "do you know", with a "duh" face or an annoyed tone: "Do you know/have any idea how many sleepless night I had to endure to make this and now you've just ruined it all?"
So which one should I choose to introduce a fact?
Best Answer
A similar question was asked to the Editor of Merriam-Webster Learner's Dictionary. Serenity Carr, Assistant Editor, answered: "The phrase 'did you know' is used when you are asking someone if they know a fact, and you already know the fact." [...] The phrase 'do you know' is usually used when you are trying to find out information you do not know."
See the full explanation and some examples: http://learnersdictionary.com/qa/When-to-Use-Did-You-Know-Instead-of-Do-You-Know-