thermal conductivity
Grab a metal spoon and a wooden spoon left in the same hot pot: the metal burns your hand while the wood does not, even though both are the same temperature. Thermal conductivity measures how readily a material passes heat along. High conductivity (metals) whisks heat away; low conductivity (wood, foam, air) blocks it.
The governing law is Fourier's law: the heat flux q (watts per square meter) equals -k x (dT/dx), where k is the thermal conductivity in W/m-K and dT/dx is the temperature gradient. A big k means a lot of heat flows for a small temperature difference. Values span four orders of magnitude: diamond about 2000, copper about 400, aluminum about 240, steel about 50, glass about 1, water about 0.6, plastic about 0.2, still air about 0.025, and foam about 0.03 W/m-K. Heat is carried by two porters — free electrons (dominant in metals) and lattice vibrations, or phonons (dominant in ceramics, glasses, and polymers).
The same k explains why metals feel cold (they pull heat out of your warm finger fast) and why foams insulate (they trap air, a poor conductor, in tiny pockets so it cannot circulate). Designers want high k for heat sinks and cookware and low k for insulation and thermos flasks. Honest caveat: feels cold is about conductivity, not actual temperature — a metal and a wooden object in the same room are equally warm; the metal just steals your body heat faster.
A copper heat sink (k about 400) moves heat away from a chip roughly 2000 times faster than the same thickness of foam (k about 0.03) would — which is why one is used to cool and the other to insulate.
Thermal conductivity spans four orders of magnitude from diamond to foam.
Metal feeling cold is not a temperature reading — it is high conductivity draining heat from your skin. In a warm room a metal handrail and a wooden one are the same temperature; the metal only feels colder.