When my kids "why" us, my husband and I generally take one of three routes:
Information - a short answer the first time, and if they ask again a MUCH longer one. For example - "Why am I drinking water? Well, when it's hot outside it causes your body to heat up, which makes you sweat and evaporate water from your body. It is important for your body to have plenty of water so that it can function properly. Without water your brain and other organs can't work right, and that can make you feel sick, dizzy, tired, or worse. You should always be sure to drink plenty of fluids when it's warm!" Honestly, I could probably even go on from there depending on which child is asking and on the question count. I've even made the child sit and look something up with me (and older children look up pertinent words in the dictionary).
"Why is there a piece of paper on the table?" Well, my answer is usually something like. "Why do you think there's a piece of paper on the table?" (to which I generally get an "I don't know") So I toss in a "Do you need to know? Is this piece of paper likely to very important to you (and if it was wouldn't you then know about the paper already)?" (usually I get a no or a not really) "Well then!" I say, "Just accept that it's there and don't worry about it" :)
Or, I simply point out that while I never mind answering legitimate questions, I DO mind being bothered with superfluous questions. Sometimes I will walk them through the logic process:
Child "Why is that piece of paper on the table"
Me: Did you just hear me say that I was leaving a note for Dad, letting him know we're going to the store?
Child: Yes
Me: Then why are you asking me? You already know the answer. I don't mind answering your questions. But, I DO mind when you haven't even tried to use your own mind to figure it out. Think about what you know, and try to figure it out before you ask. If you can't then fine, ask me - I'm glad to help - but you do need to try to answer your own questions first. I won't always be there to tell you things, and you have to learn to figure the answers out on your own.