The answer is never, that is, on a 2 dimensional plane. You can perform an experiment to see why it is the case. On curved surfaces though, two lines can intersect one another more than once. For instance, on the surface of planet Earth, two lines can intersect one another, both at the Earth's North Pole and South Pole.
each trial is independent so do
.5 x .5 x .5 or .5^3
.125 is the probability.
