It's a three-block structure:
On the top, you place a block that falls down ( sand or gravel)
In the middle, a solid regular block (such as wood or stone)
In the botton (at floor level), an empty block.
When you want to pass, destroy the solid block, and the top block will fall down and fill the empty space at the bottom. Thus, a door.
Actually, this already exists in McZall, McZall and McForge softwares (If I missed any, tell me). They're called Tdoors, and they allow active materials to pass through when activated.
--> [Any non-physics block]
<--no grass here
It's a three-block structure:
On the top, you place a block that falls down ( sand or gravel)
In the middle, a solid regular block (such as wood or stone)
In the botton (at floor level), an empty block.
When you want to pass, destroy the solid block, and the top block will fall down and fill the empty space at the bottom. Thus, a door.