Real Number is Limit Point of Rational Numbers in Real Numbers

Theorem
Let $\R$ be the set of real numbers.

Let $\Q$ be the set of rational numbers.

Let $x \in \R$.

Then $x$ is a limit point of $\Q$.

Hence the interesting case of a $\Q$ is countable set $\Q$ whose set of limit points is uncountable.

Proof
Let $\epsilon \in \R_{>0}$ be arbitrary.

Consider the open interval $\openint x {x + \epsilon}$.

Then by Rationals are Everywhere Dense in Topological Space of Reals, there exists a rational number in $\openint x {x + \epsilon}$.

Hence the result by definition of limit point.