Subject: What criterion must be met to define something as a living thing?

My 6 yr old believes he has discovered, "rocks are alive"! His 
experiment involved placing rocks in water and watching little air 
bubbles rise to the surface, "they really breathe!".  My explanation 
is not convincing him, so I am looking for criterion that needs to be 
met to define a living being, i.e. respiration? growth? reproduction?
He has considered rocks may reproduce by weather - wind and ice 
breaking little pieces off.  He has learned that sedimentary rocks 
form when smaller rocks "grow" into bigger rocks under great pressure.
Simon loves science and I would like to turn this into a lesson of 
making a hypothesis and going through the steps to prove it right or 
wrong.  Thank you for any assistance you can offer.  Jana

Re: What criterion must be met to define something as a living thing?