My multi-purpose sanity-saver answer was "What do YOU think the answer is?" I think a lot of times a curious, inquisitive, chatty child really wants to talk and asking the question over and over again is their way of engaging in conversation, so if you turn the question back on them, often times they'll happily take the opportunity to tell you all about what *they* think :-).
Another useful phrase is "let's look it up next time we're at the library" (yes, I know you can get instant answers off the internet, but I'm still sorta old school in believing that looking stuff up in regular books is a useful skill and something to encourage in children - also it means that I have deferred answering the question until our next trip to the library!)
And hang in there - pretty soon your daughter will be able to pepper those questions to her teachers at school so you won't have to deal with them all yourself (one of my 9YOs is an inquisitive perpetual-question machine and I find she doesn't exhaust me quite so much now that she's absorbing so much more info at school)
Edited to add: be forewarned, though, that once your daughter starts looking up answers on her own, if she's anything like my factoid-junkie 9YO, you'll soon be hearing a lot of "Mom, did you know that......." ;-) (BTW my stock answer for *that* situation is usually "No, not until you just told me!")