Here's why:

* Thermal conductivity measures how well a material conducts heat. Materials with high thermal conductivity allow heat to flow through them quickly and easily.

* Metals are generally excellent conductors of heat. Examples include copper, aluminum, gold, and silver.

* Other good conductors include diamond and graphite.

* Materials with low thermal conductivity (like wood, plastic, and air) are called insulators and resist the flow of heat.

In summary: The better a material conducts heat, the easier it is for heat to flow through it.