Well you know when you do the finger RC to reaffirm you are dreaming, you don't think "will my finger go trough my hand." You know your finger will go through you hand.
And you have to apply the same logic to glass, or walls or anything - knowing you can do it, and knowing it's a dream, allows you to easilly achieve this, and I usually start by just poking my finger through, much like the finger RC and then just put my hand, arm, and then everything though the object.
Try it with people too

Bookmarks