The hard questions just keep coming don't they. I am so proud of the attitude my kids have, especially the 5 yr old, and I think it comes from always speaking to them as if they are little adults. We always answer questions properly and tell them we will find out if we don't know (more often now we don't!). Obvoiusly we are slightly more detailed with one than the other and we keep our answers age appropriate but I think generaly if you stick to the simple truth then you can't go wrong.
My son (aged 5) asked me the other day "If cave men were the first people on Earth then who were their parents?" Erm...erm...