The thermalization of two blocks with distinct initial temperatures put inside an insulated
recipient is an irreversible process to which the entropic version of the second law of
thermodynamics applies, so the entropy of this system must increase. In a very recent note in this
journal, Mungan (2015 Eur. J. Phys. 36 [http://dx.doi.org/10.1088/0143-0807/36/4/048004] 048004 )
has presented a proof of such an increase in the more general case of distinct blocks, but his
approach involves the analysis of some series expansions, which sounds mathematically complex for
first year undergraduates. His approach also has the disadvantage of demanding the analysis of
distinct cases, according to the values of the initial temperatures and thermal capacities of the
blocks. In this short note, I make use of the concavity of the logarithm function to present a
simpler, elementary proof which is more suitable for introductory physics classes.