I have resigned myself to realize that even the most rigorous college science training does not prepare one for the questions 7th graders ask. "What state of matter is fire?" (hot gas). "Are there siamese-twin animals?" (there are). "How do bears pee when they're hibernating?" (they don't. they recycle the nitrogen into protein). "What would happen if you dropped antimatter in the earth's mantle?" (I don't know).
You'll notice none of them ask me what the Na/K ratio is in the sodium-potassium pump. Or what is the adductor to conteract the deltoid muscle. They don't ask me how many electrons are in the outermost orbital of a noble gas. Nope, my college education does not help me when kids start asking questions.
But, what does help me is a pad of post-its, an insatiable desire to learn, and access to the internet. At the end of the day, when my computer screen is littered with question-riddled post-its, a few students and I will look for the answers, and those we can't find, we submit to the experts. My favorite site is the UCSB Scienceline, where actual scientists email responses to student questions, although Wonderquest and The Why Files have helped us out, too. And I can't leave How Stuff Works off the list of most helpful sites.
"When you know a thing, to hold that you know it; and when you do not know a thing, to allow that you do not know it; this is knowledge." - Confucius
"The greater our knowledge increases, the greater our ignorance unfolds." - John F. Kennedy
"The more I learn, the more I realize I don’t know." - Albert Einstein
"The only true wisdom is in knowing you know nothing." - Socrates
These are some pretty smart guys, and they seem to be sharing a similar message. Ignorance itself is not to be feared. It is an tool to define the parameters of your current knowedge and to identify areas in which to expand your knowledge.
I firmly believe in admitting when I don't know something. In fact, I think it is very powerful to say to a student, "I don't know," followed by, "but we can certainly find out!"