Shortest Path on Cube

An ant is sitting on the vertex of a cube. What is the shortest path along which it can crawl to the diagonally opposite vertex? The ant stays on the skin of the cube all the time.

Here is a solution presented by the students in class:

• Open the cube (flatten it up)
• Join the two points by a straight line on the surface of the flattened figure
• Then re-make the cube again