How to succesfully implement OnCollisionEnter?

I’m pretty sure I’m missing something very simple but, alas, my line of sight is blurrying my vision here.
Please help me.

In order to show my problem in a simple scenario, I created a new scene.

Added a simple 3D Cube, clone it and move it near the original.

Inspector cube a:
http://cdn1.npaste.com/image/original/1470805079/obogfk.jpg

Inspector cube b:
http://cdn1.npaste.com/image/original/1470805127/oboghe.jpg

Attached a simple C# behavior component to Cube b:
http://cdn1.npaste.com/image/original/1470805171/obogix.jpg

Press play and move the cubes so they can kisskiss:
http://cdn1.npaste.com/image/original/1470805236/obogk4.jpg

Nothing… Needless to say, I feel very hmm… limited (not to say super stupid) in how to implement a simple collision detection.

I came across with this problem at attaching a similar script to a fired rocket towards some asteroids.

Any ideas of what am I missing here? Thanks in advance.

Attach a rigidbody to the cube. Hope it helps.

open this : Unity - Manual: Introduction to collision
and scroll down to Collision action matrix