Simplify. \[ 9 \sqrt{18} \]

To simplify \( 9 \sqrt{18} \), we start by breaking down \( \sqrt{18} \). We know that \( 18 = 9 \times 2 \), so: \[ \sqrt{18} = \sqrt{9 \times 2} = \sqrt{9} \times \sqrt{2} = 3\sqrt{2}. \] Now, substituting this back into the original expression, we get: \[ 9 \sqrt{18} = 9 \times 3\sqrt{2} = 27\sqrt{2}. \] Thus, the simplified expression is: \[ \boxed{27\sqrt{2}}. \]