Closure Properties of Reals

A set is said to be closed under an operation if any two elements of the set produce another element of the same set. So, for example, if you add $\frac{1}{2}+\frac{1}{3}$, you get $\frac{5}{6}$, which is still a rational number. So you can say the set of rational numbers is closed under addition. Which operations is the set of rational numbers closed under?