The Green Glass Door is a common word game where a player already introduced to the game (understands the 'trick' to it) gives clues to a new player about what does or does not belong behind the 'green glass door'.
All the clues will have words that have exactly 2 duplicate letters which belong inside the green glass door. It is better to have the two items mentioned related in some way to greater confuse the individuals participating in the riddle.
The reverse of this game is known as the Red Plastic Window, in which each word with no double letters is behind the Red Plastic Window.
Other common variations of the Green Glass Door are Moody's Grill and Bobby's World. Things with double letters are said to be in Moody's Grill, or can be taken to Bobby's World, other things are not. In England, a common variation is talking about what Harry the Hippo does (double letters) and does not like.