This riddle asks: What kind of nails never need a hammer?
The answer is: fingernails.
The word “nails” usually makes people think of the metal fasteners used in construction. Those nails are driven into wood or other material with a hammer.
But fingernails are a completely different kind of nail. They grow naturally on your fingers and never need to be hammered into anything.
The joke works because the same word can describe two very different things.
Once you move away from tools and construction and think about the human body, the answer becomes obvious. It is a short and familiar riddle built around a simple double meaning.
