Simplify $16^{\frac{1}{2}}-625^{\frac{1}{2}}$.
I recognize that the exponent of $\frac{1}{2}$ means taking the square root, so I can rewrite the expression as $\sqrt{16}-\sqrt{625}$. Now I can evaluate the square roots of these perfect squares: $\sqrt{16}=4$ and $\sqrt{625}=25$. So the expression becomes $4-25$, which is just $-21$. 