1. Why do liquids and solids have a relatively fixed volume (subject to small expansions and contractions due to temperature), whereas the volume of a gas is much more variable?
   
   
2. Why do crystalline solids have a fixed shape, whereas liquids and gases adapt to the shape of their containers?
   
   
3. What is different about the way that liquids and gases adapt to their containers?
   
   
4. What holds the molecules of a molecular liquid or solid together? Why doesn't this same factor hold for gases?
   
   
5. What were the earliest two chemical elements?
   
   
6. Why are these two elements so much rarer on Earth than they are in the universe as a whole?