Re: Why is a desk solid, and why doesn't it evaporate, or melt in under normal conditions?

Well, actually, your desk IS evaporating but at a very slow rate.  A 
little background on definitions:  Melting is a process of going from a 
solid (like your desk) to a liquid.  Evaporation is going from a liquid to 
a gas (or vapor - same thing) and sublimation is going directly from a 
solid to a gas without ever being a liquid.

Pretty much everything can be made to melt or evaporate.  Only some 
materials can be made to sublime.  All that is needed is enough energy 
(which we usually apply as heat) to overcome the forces that hold the 
molecules of the material together.  

Now, when some materials are heated, they decompose before they can be 
heated enough to melt, evaporate, or sublime.  Wood is a good example - if 
you heat it enough, it either starts to burn or just turns black. Other 
materials, such as metal can be heated until the forces holding them 
together in a rigid structure are overcome.  Then the individual molecules 
can move apart and the material is seen to melt to a liquid.  Additional 
heat gives the molecules enough energy to "fly" out of the liquid and 
evaporate into a gas.  (In sublimation, molecules remain a solid until the 
gain enough energy to pass into a gas directly).

The fact that you don't see solids evaporate or melt before your eyes is 
only due to the fact that we live in a pretty mild environment.  At any 
time, a few molecules on your desk probably have enough energy to "fly" 
out of the solid into a gas but it is too few to notice.

BUT, if you were sitting on the surface of the Sun, there would be so much 
energy that the molecules in the solid of your desk would quickly melt 
then evaporate, or sublime (depending on the material).