Re: 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


Dear Jana,


I would try my best to explaine this to your son, but if this is not yet 
convincing, please feel free to contact me again so that I can find out a better 
answer.


In my opinion there are two most important criteria to define life.  First is 
reproduction and the second is metabolism, i.e., the intake of nutrients and the 
output of waste.


Reproduction can take the form of either sexual or the asexual way.  Sexual 
reproduction involves the combination of germ cells from a male and a female, 
while the asexual reproduction usually happen with merely dividing of the mother 
cell.  A rock cannot reproduce independently.  If you break up a rock it will 
not finally give rise to rocks just like itself.  If you put two rocks together, 
they will not give birth to a new rock which will finally grow up to be similar 
to the parents. 


Metabolism is closely related but not limited to growth.  All living things need 
to take in something to maintain their order inside of them, their weight, and 
their mere being.  Rock does not.  If you isolate rock in a cabinet, it 
basically does not lose weight.  Intake of oxygen and exhale of carbon dioxide 
is part of the metabolism too.  You might want to tell your son that the bubbles 
of the rock is just caused by the escape of air stored inside of the hollow 
caves of the rock, while his own breathing is chemically much more active.  The 
gas he takes in is different in composition from the gas he breathes out.  
Experiments can be carried out to show that but I don't know whether his school 
lab can do that or not.


I think the hard thing to do here is to convince him that grass is living but 
rock is not.  In this case you can tell him that grass will die if you don't 
provide it with water...and even you do it does not live forever while rock is 
almost immortal.  I would like to share with followup questions if he has 
further doubts.


It is always a pleasure to help young minds pursue the dream of science.  I am 
really happy that you are such a good mother to help him understand what he 
likes.


Best regards,