The riddle
What Room Has No Windows or Doors?
Find the room inside the answer
Split the answer into "mush" and "room" and the riddle opens up. Listeners picture a closet, a cellar or a vault and search for a way in. The question names a room, and the answer is a word that contains one. A mushroom has no windows or doors because it is not a room at all, only a word that ends in "room".
What a mushroom is
A mushroom is the fruiting body of a fungus, the part that makes and releases spores. Most of the fungus stays out of sight as a mat of fine threads called mycelium, living in soil, wood or fallen leaves. Fungi are not plants. They form a kingdom of their own and do not make food from sunlight. A typical mushroom has a cap on a stalk, and many kinds carry thin gills underneath where the spores form. The mushroom that appears above ground is only the visible piece, and it shows up when conditions suit it.
A word of caution
Mushrooms found outdoors are for looking at. Some poisonous kinds closely resemble safe ones, and even experienced foragers can be fooled, so wild mushrooms are best left where they grow.
Hunting for hidden words
The same search works on other words. "Carpet" hides "car" and "pet", and "classroom" holds "room" again. A good way to pass the time in a car is to take turns spotting a short word inside a long one, which is exactly the skill this riddle needs.