Open Sets in Indiscrete Topology

Theorem
Let $T = \left({S, \left\{{\varnothing, S}\right\}}\right)$ be an indiscrete topological space.

Let $H \subseteq S$.

$H$ is an open set of $T$ either $H = S$ or $H = \varnothing$.

Proof
A set $U$ is open in a topology $\tau$ if $U \in \tau$.

In $\tau = \left\{{\varnothing, S}\right\}$, the only open sets are $\varnothing$ and $S$.