[en] The two main properties related to heat conduction are thermal conductivity (K) and thermal diffusivity (D). However, while the meaning of K is very well known, the role played by D is usually undervalued and misunderstood. In this paper we will try to clarify the meaning of thermal diffusivity, first in homogeneous materials and then in composite materials. In this latter case, we will find that sometimes the association of two good thermal diffusers gives a bad diffuser. Moreover, by properly choosing the constituents of the composites we can manufacture materials with thermal properties that are not found in nature