7. Take four points $A, B, C, D$ randomly on the circumference of a circle, then the probability that line segments $AB$ and $CD$ intersect is $\qquad$.


7. $\frac{1}{3}$.

Solution: Take any four points on a circle, label them as $A, B, C, D$. There are 24 ways to label them, and 8 of these ways result in $AB$ intersecting with $CD$. Therefore, the probability is $\frac{1}{3}$.