I have done BlenderGuru's Quick Rigid Body Fun on Rigid Body Collisions.
Regarding this .blend file:
When you play the animation, the cubes start falling down long before they are hit by the wrecking ball. This is because of gravity and because there is some space between the cubes. How can you prevent the cubes falling down or at least arrange that the cubes don't fall down before they are hit by the wrecking ball?
P.S Blender Guru says it is necessary to have some space between the cubes because if there is no space the wall will explode. I have here another .blend file...
...where there is no space between the cubes, so I don't think he is right. But the stones don't behave like there is a wall .... hmmmm ???