Thermal Conduction is the main way heat is transferred through solids. Metals are excellent conductors of heat whilst liquids and gases are poor conductors of heat.
The videos below demonstrate conduction of heat through two different metals: Copper and Steel. Ballbearings are attached, using small amounts of wax, at regular intervals along two metal rods, which are heated where they meet in the centre.
The first video is a shortened (edited) version of the second, full length, video.
Full length video:
Explanation of video:
The molecules within a solid are in a constant state of (thermal) agitation: Whilst molecules are locked in place they constantly vibrate. The temperature of a solid is related to the size of these vibrations: The hotter a solid become the larger the vibrations.
When a solid is heated the molecules in the heated part vibrate more. These vibrations are then transferred from molecule to molecule throughout the solid. This process is called Conduction.
Metals gain their excellent thermal conductivity from their free electrons which, by colliding with the ions within the metal, help to transfer the vibrations more rapidly. Copper, being an excellent electrical conductor, has more free electrons per ion than steel and hence more electrons to help with the conduction of heat.