  For the vectors $x$ and $y$ in a normed linear spaces $X$, the mapping
$n_{x,y}: \mathbb{R}\to \mathbb{R}$ is defined by $n_{x,y}(t)=\|x+ty\|$. In
this note, comparing the mappings $n_{x,y}$ and $n_{y,x}$ we obtain a simple
and useful characterization of inner product spaces.
