Frustrating People

I had to deal with a rather irritating customer at work today.
Which was made slightly worse by the fact that the store had barely been open 30 minutes when she walked in the door with an attitude that didn't bode well.
The next twenty minutes were spent with this customer with me quietly growing more frustrated with her because she was acting like she expected me to know nothing about the products that she wanted to know about and wanted me to go get the 'expert.' which considering how long I've worked at the store....I'm basically the expert.

But when I get asked weird questions like: 
"Does this hairball preventative gel have calories in it? How many?" 
or "Can you recommend something to give my cat so that he'll drink water?" 
"Is this litter scented or unscented that comes in the disposable trays?" 
"I don't care what brand of food it is I just need to find beef flavored cat food where is it?"

Then I'm totally thrown for a loop.
Because a) they're weird questions and b) why in the world would it matter? or c) how did you even think there would be something like that?

It's like....I know the answers to a lot of things, but because you asked me questions I don't quite have the answers to, it makes me feel dumber and look dumber in your eyes because I 'don't know everything.' 

XP bleh. 
I could totally feel a tension headache coming on because this woman was sooo difficult to deal with and help because she was very particular in what she thought her needs were, but she wasn't giving me enough detail to be able to adequately help her meet those needs.

Thankfully, a little bit of Excedrin taken after that encounter helped to head off the headache I could feel forming.
Because I did not want to have to suffer through an 8 hour shift because of one customer.