howstuffworks.com is an excellent site to look it up. i think when my kid starts asking such questions...i would be very proud! it means they are thinking and asking why, rather than take things as us adults have layed them out. i would encourage it rather than descourage it so that they lose interest!
if you are not close to the internet...tell them you will look it up at night and explain to them. get them close to teh computer or study it beforehand and then explain to them using pictures. you can foster their interest in science, which too many of us lose in the beginning. their loss of interest is definately not better!!
i can understand often times explanations are difficult and not layed out as kids....in that case go to the library and look up kids books. but kids are use to us providing explanations of things that they don't understand...thats how they learned to talk! we use sooo many different words in every situation, yet they still figure it out. sooner or later your explanation will stick in their head also and they willl figure it out!