  Here we briefly resume the idea, originally introduced in Phys. Rev. D 102,
106002 (2020), that the Bekenstein bound on entropy is a consequence of the
fermionic nature of fundamental degrees of freedom, which arrange themselves to
form matter and spacetime. The main point is discussed by means of a toy-model
of black hole evaporation, which describes the dynamics of such degrees of
freedom, called Xons. An intrinsic notion of interior/exterior of the black
hole during the evaporation process is given and both von Neumann and black
hole/environment entropies are computed.
