In a book stands that rational algebraic expression is $\displaystyle {\sqrt 2}$ and that are not rational algebraic expression $\displaystyle {\sqrt 2gh}, x+{\sqrt x}$

How can $\displaystyle {\sqrt 2}$ be rational algebraic expression? Why isn't irrational?

Why $\displaystyle {\sqrt 2gh}, x+{\sqrt x}$ are irrational?