Do you want to walk through walls?
Do you need secret entrances?
Do you want a good hiding place?
If so read on.
Step 1: Dig a Entrance and Room
Make sure to dig a cave sort of entrance/room in other words make sure it has a roof.
Step 2: Make Signs
One block before the front of the cave/room build signs one on top of the other like in the picture.
Step 3: Cover the Signs With Paintings
Now place one painting over each sign like on the picture.
Step 4: Now Walk Through It!
There you go! This trick can be modified to suit many different rooms,caves,etc
Thanks hope you like it!