The riddle
Why Do Cows Wear Bells?
A horn that makes a sound
The word horn has two meanings. One is the hard growth on the head of an animal such as a cow. The other is a device that makes a loud sound, like the horn on a car. The riddle says the cows wear bells because their horns "do not work", meaning they cannot honk. A cow's horns were never made to sound, so the bell covers the gap. The riddle is best told aloud, because "horn" has to be heard in both senses at once.
The real reason
Herders hang bells on free-roaming livestock so they can find an animal by ear when it grazes out of sight, as in hilly country or on wide plains. Bells are common in Europe and Latin America, and herders in Africa and Asia use them too. Different bells sound different, and the sound can indicate an animal's species, sex and age. On Alpine pastures the sound helps farmers find cattle that have gone behind a hill, into thick plants or into mist. In Switzerland some cows wear especially large bells when the herds come down from the summer pastures in autumn.
Bells that clank and bells that ring
Construction changes the sound. Swiss trychlen are made of hammered sheet metal, so they clank instead of ringing crisply like cast-metal bells, but they are lighter to carry.
More farm wordplay
Another farm riddle that turns on a double meaning is why did the scarecrow win an award.