This simple-looking riddle asks: What can you never put in a refrigerator?
A common joke answer is: the refrigerator itself.
The trick is that most people immediately start thinking about foods that should not be chilled. But the riddle is not really about food at all. It is asking you to think about the refrigerator as the object in question.
In the ordinary sense, you put things inside a refrigerator, but you do not put the refrigerator inside itself.
Like many short riddles, the wording is deliberately broad so your mind jumps to complicated possibilities. The joke works by bringing the answer back to the most obvious object in the question.
