Subject: What causes different types of materials to conduct heat at different rates

To Whom It May Concern;
     My son is currently doing his 7th grade science project about heat
conduction. Through experimentation he has shown that different materials
conduct heat at different rates. He now needs to do research as to why this
phenomenon occurrs. Is there any information that you can offer (which will be
understandable at his level),
to help him comprehend this process?
Also if you can reccommend any online or offline reference sources which will
aid his research, it would be greatly appreciated. 

Re: What causes different types of materials to conduct heat at different rates