I always wanted to know why Orange Juice tastes so bad after you've brushed your teeth...which led to me an awesome website which I last read a couple years ago; HOWSTUFFWORKS. Did I find the answer? Sort of - it states in the opening para; "In actuality, scientists don't know for sure why orange juice and toothpaste go so poorly together. This is mostly due to the fact that scientists aren't entirely certain how taste works. Like smell, full understanding of this sense remains elusive."
Pretty interesting and got me checking out the rest of the site - AWESOME information and definitely a website to tell your kiddies about! (Actually; keep it a secret and use it whenever they ask that really difficult stuff...)
Note: They've also embedded video search - e.g. wanna know how a helicopter works? Click here to learn.