The rule implies that in a physically meaningful expression only quantities of the same dimension can be added, subtracted, or compared. For example, if m man, m rat and L man denote, respectively, the mass of some man, the mass of a rat and the length of that man, the dimensionally homogeneous expression m man + m rat is meaningful, but the heterogeneous expression m man + L man is meaningless. However, m man / L man is fine. Thus, dimensional analysis may be used as a sanity check of physical equations: the two sides of any equation must be commensurable or have the same dimensions.