You say that you ask “why do crystals do what the do” but what is it that a crystal actually does ?

Question: You say that you ask "why do crystals do what the do" but what is it that a crystal actually does ?
- Keywords:
  crystal
Asked by justcallmex to Andrew on 22 Nov 2011.
- Andrew Cairns answered on 22 Nov 2011:
  
  The crystals shrink when we heat them (almost all materials expand) and they expand when we put them under pressure (everything shrinks normally).
  
  These effects are called ‘negative thermal expansion’ and ‘negative linear compressibility’ and are extremely rare.
  
  Hope this answers your question!

Question: You say that you ask "why do crystals do what the do" but what is it that a crystal actually does ?