SOLID TEMPERATURE CHANGE

We humans can't detect the motions of the particles in matter, but they are moving.  All particles at any temperature are moving, even really cold particles.  The animations below are a representation of four particles of a solid.

Cold particles

When particles are in the solid state, they are still in motion.  They just move slower, and not very far.

Warmer particles

When these particles are heated, they move faster and farther.  This explains why most materials expand when heated.  Notice that the particles remain the same size.  If these particles gained enough energy, then they could break out of their positions and slide around past each-other and the solid would melt.
Since the particles of a liquid are moving more than those of a solid, the volume of the liquid is a little higher than that of the solid...  this means that if you have a partially melted solid, then it will sink to the bottom of its own liquid.


There is a notable exception to this tendency:  when liquid water becomes solid

When water particles slow down, they line up in a certain way.  This crystal structure is actually a little bigger than the same particles were when they were in the liquid state, so solid water floats on liquid water.
Water is VERY unusual in this respect.  Most materials do not behave this way.
Imagine if water were like most other materials...  ponds would freeze from the bottom up!