This has been one of my favorite parts of being a parent. Messing with a a kid's head is truly a wonderful thing.
It started with "Why is the sky blue?", and my response being "Because that's the color I wanted it to be." The same with grass being green as well.
My all-time favoritte though was when my oldest daughter was about 12 or 13. She was bragging about doing something and so I sarcastically said how special she is, to which she agrred she was. Then I said she was so special she had her own Olympics, the Special Olympics. She once again agreed, not knowing what the Special Olympics really were.
The next day she went to school telling all her friends how special she was and how she had her own special olympics. They let her in on what exactly the Special Olympics were.
7 years later she still bears a grudge about that one