How do i make fake blocks?

[gravitygoesup1]

Okay i use editroid and i'm a noob so I don't know which blocks are air or solid or how to make them. I want to make the secret passages like in Norfair in M1 and I know you can shoot and bomb crumble blocks, but I want fake blocks. Any help would be appreciated.

[jamma]

what do you mean by fake blocks? Like a block that appears to be solid yet you can pass right through it as if it's air?