This is Bertrand's paradox. For a short introduction, I advise you to watch the video here by 3Blue1Brown and Numberphile. In short, Bertrand's paradox asks: "Let us pick a random line segment of length $\ell$ inside a unit circle. If we inscribe an equilateral triangle of length $s$ inside the circle, what is the probability that $P (\ell > s)$?" The paradox comes because there are three different ways to solve this problem which comes up with different probabilities. In the following simulation, I colored lines that were shorter than $s$ with dark cyan, and I colored lines that were longer than $s$ with azure. The probability approaches $\frac{1}{3}$.

Probability: 0