The riddle
What Is So Fragile That Saying Its Name Breaks It?
Say it and it is gone
Silence is the absence of sound, as Merriam-Webster defines it. To say the word "silence" is to make a sound, so the moment the name is spoken, there is no silence left to name. The riddle calls it fragile because it cannot survive even the smallest interruption. The word can also mean a refusal to speak, as in staying silent about a secret (Merriam-Webster), but the riddle uses the plain sense of quiet.
Why a sound needs something to travel through
NASA explains that sound is a vibration that must pass through a medium such as air or water. In the near-vacuum of space there is almost nothing to carry it. On the ground, air is everywhere, which is why even a quiet room has faint noise.
In a 1924 book
Lawson's 1924 collection of conundrums asks, "What is it that is so brittle that even to name it is to break it?" The printed answer is "Silence." The only change from today's wording is brittle in place of fragile, two words with close meanings.
Trying it in a room
Ask the riddle in a quiet room and wait. Listeners often guess glass or ice, and each can be tested against the clue about the name. Glass does not break when someone says "glass". Only silence ends the instant its name is spoken. A good closing move is to let the room go quiet again and ask for the answer in a whisper. Some tellers hold a finger to the lips instead of saying the word, so the group gets the answer without breaking anything.