Re: Why do the particles in a solid stay together?

Good question.  You are right - if there were not some forces holding the 
particles together, gravity would pull them apart and all solids would 
collapse.  So there obviously must be some forces holding the particles 
together.  These forces can be chemical, electrical, or mechanical.

Mechanical forces can hold particles together by friction or interlocking 
edges, for example.  A large scale example of friction would be how rough 
bricks can be stacked into a jumbled heap without the whole pile 
collapsing.  Interlocking particles are similar to the way jigsaw puzzle 
pieces connect and hold each other in place.  These forces can apply on 
microscopic to large scale depending on the size and shape of the 
particles.

Chemical forces also hold solids together.  Depending on the material, 
there are variety of different kinds of chemical bonds that can form 
between the individual particles.  It all depends on how the electrons in 
the individual molecules interact.  

Electrical forces can hold particles together by the attraction between 
positively-charged particles and negatively charged ones.

Which forces are most important in a particular solid depends on the 
materials (materials) involved, the size and shape of the individual 
particles, and the conditions (like temperature and pressure).  It is the 
combination of the various forces and conditions that given solids their 
differnt properties.