I've had to answer _some_ questions for my nephew (who's 10.5
  now) through the years, but I'd make them quick and applicable
  and get the questions out of the way as fast as possible because
  I didn't want to make it a "THIS IS A LEARNING OPPORTUNITY
  MOMENT", because I'm not the parent and it's not my place.
  Certainly nothing close to the level of having to describe an
  itch in a place you'll understand when you're older. More like:
  [6 yr old question] "Why does my pee go up instead of down when
  I go to the bathroom?" Uncles answer: "your pee goes up instead
  of down sometimes because there's extra blood in your weiner
  that makes it stand up instead of down. It's normal. It's harder
  to pee like that, so just wait a few minutes and it'll go away
  on its own and then you'll get the pee in the bowl and your mom
  won't yell at you". "Oh!" "Did that make sense?" "Yeah, it makes
  sense. I'll just wait a few minutes next time". There. Done. ==
  [actually come to think of it, what I did is _precisely_ a "THIS
  IS A LEARNING OPPORTUNITY MOMENT" - except I wasn't thrilled
  about it... more like, "Let's be accurate, thorough and get this
  over with as fast as possible, Ken..... think quick quick think
  quick... ok GO!"