Be $m$ and $n$ two perpendicular lines, and be distinct points $A$ and $B$ outside the lines and in the first quadrant. What is the shortest way to get from point $A$ to point $B$ by tapping the two lines?
Idea: Listen say that the shortest path between a point and a line is perpendicular to this line, but I'm not convinced. =S
Use reflections and the principle that the line segment is the shortest unrestricted path: