When thermal energy is exchanged from one particle to a nearby one by means of a collision or a movement that induces movement (for example, through electrical attraction or repulsion), the process is called conduction. Though it’s the most straightforward means of heat transfer in matter, conduction takes different forms in different materials. For example, metals are usually good conductors of heat because, while their atoms are fixed in a lattice-like structure, some of their electrons are very loosely held and tend to form a free-moving “fluid” or “gas” in the solid that can carry energy from one region to another. This same electron mobility makes metals good electrical conductors. But in nonmetallic solids like ceramics, conduction is more mysterious. It seems that heat is propagated not by the movement of energetic electrons—in solids of ionic- or covalent-bonded compounds, the electrons are not free to move—but by the vibration of individual molecules or a portion of the lattice, which is transferred to neighboring areas. This transfer of vibration is a much slower and less efficient process than electron movement, and nonmetals are therefore usually referred to as thermal or electrical insulators, rather than conductors. Liquids and gases, because their molecules are relatively far apart, are very poor conductors.