Consider the L-shaped brick.Suppose an ant start from $A$ and it wants to go to $B$. It can only travel on the surface of the brick. What is the minimum distance that it has to travel??
I tried to open the box and make it a 2D figure but failed. I tried to bash it but also failed. Do you have any idea?
Convince yourself that the shortest path between $A$ and $B$ is entirely visible in the picture, and does not travel along the leftmost visible square, shaded red in the following picture:
Fold the top face $F$ up along the blue line so that it is coplanar with $G$ to see that the surface that the shortest path is on is symmetric. So by symmetry there is a shortest path that does not travel along face $G$ or the squares adjacent to it. This means there is a shortest path that only travels along the top $L$-shaped face and the face containg $B$. This you can fold flat to easily see such a shortest path.