Basically, 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.
Behind the door you may attend classes at school or college but not courses at a university.
Minnie Mouse is in, Mickey Mouse is not.
Stellar is in, lunar is out, but moon is in and stars are out.
Buttons and zippers are behind it, snaps aren't.
Flowers may bloom there but they don't grow there.
There is Bill, but no Clinton. Chelsea will have to wait outside.
The U.S.S.R. is in, but the Union of Soviet Socialist Republics is not.
In a similar vein:
A stool is behind the green glass door, but a keyboard is not. (no relation)
Matresses are behind the green glass door, but a speaker is not. (no relation)
There are books but no shelves
There is a moon but no sun
There is hell but no heaven
There are bunnies but no hares
There is killing but no murder
There is kissing but no love
There is Google but no internet
There is affection but no love
There are streets but no roads
There are pools but no water
There are butts but no chairs
There are feet but no shoes
There are roofs but no ceilings
There are floors but no tiles
There are bullets but no gun
There is a hammer but no nails
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.
There are animals but no sheep
There are lifeguards but no swimmers
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.
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