Interior of Singleton in Real Number Line is Empty

Theorem
Let $\struct {\R, \tau_d}$ be the real number line with the usual (Euclidean) topology.

Let $a \in \R$ be a real number.

Then:
 * $\set a^\circ = \O$

where $\set a^\circ$ denotes the interior of $\set a$ in $\R$.