The riddle
What Can Go Through the Green Glass Door?
The rule
Anything whose name has a doubled letter can go through the green glass door. Green has ee, glass has ss and door has oo, which is the clue hiding in the name. A balloon can go through, since it has ll and oo, and so can coffee, with ff and ee, or an apple, with pp. A window cannot, nor can a chair or a pencil, because none of them has two matching letters side by side.
It is a word game
This is less a riddle than a guessing game with a secret rule. The leader says what can and cannot go through, the players offer their own examples, and the leader answers yes or no. The rule is about spelling and not about meaning, and the players spend a while thinking about green things, glass things and doors before they see that.
Letters, not sounds
The rule counts letters, and only when the same letter appears twice in a row. A banana repeats its a and n, but never side by side, so it stays out. Cheese goes through because of ee, but cheap does not, even though both words use the long e sound. The word knee goes through, and so does a puppy, with its pp.
Keeping the secret
Start with three or four statements such as "a carrot can go through but a potato cannot". Let players call out guesses and say yes or no without a reason. Resist hints for several rounds. When the group is stuck, say that the answer is in the spelling and nothing else. Anyone who sees it should show that by naming a new example and not by saying the rule aloud.