Four guard towers are situated in a square formation of side length 1km. A general wants to build roads to connect the towers so that one can walk from any tower to any tower along the roads, possibly passing through towers. The size of the towers is too small to matter; think of them as points.
What is the smallest total length of road that suffices? For example, one can build roads along three of the sides of the square for a total length of 3km, but better is possible.
Stating it mathematically, your goal is find a finite collection of line segments with total length as small as possible such that their union is connected and contains points $(0,0),(0,1),(1,0),(1,1)$.