Tonight, after supper, the kids and I were outside.  Will noticed the moon.

"Look Mommy! The moon! How do people get there?"
"In a really powerful rocket. Or the space shuttle." (We have a book on Space he likes to read.)
"But Mommy, if people went to the moon they could knock it down, and break it."
"Um, no, the moon is big, and can't be broken. The ship can land on it and people can walk on it."
"Oh."

…A few moments later…

"Mommy, why is the moon like that and not round?"
"Well…(oh crap!)…The earth's shadow is covering up part of it. blah, blah, sun, then earth, then moon, blah, blah …"
"Well, the clouds can cover the moon for that shape too."
"Um, the clouds are here, the moon doesn't have clouds."
"Why?"
"I don't know, it just doesn't."
"Ok. I'm going to go play now!"
"Ok!" (Thank goodness!)

I was a BIOLOGY teacher, not physical science one!