  We give a conceptually simple and essentially one-dimensional approach to
Rellich inequality in Euclidean space $\mathbb{R}^n$. In particular, we show
that the radial part and the spherical part of the standard Laplacian form an
angle in $[0,\pi/2]$ when $n\geq4$, a property known in the works of
Evans-Lewis, Machihara-Ozawa-Wadade, and Bez-Machihara-Ozawa. Our proof here is
direct and short.
