Why do cows have bells?

The joke “Why do cows have bells? Because their horns don’t work!” is a form of wordplay that uses the double meaning of “horns.” Typically, cows are known for their physical horns, which are hard, permanent parts of their head. However, “horn” is also a colloquial term for a vehicle’s horn, which is used to make noise and alert others. In the context of the joke, “horns” refers to this second meaning. Since cows obviously can’t honk like a car, the joke humorously suggests that they wear bells as an alternative to make noise, since their “horns” don’t function like a car’s horn to “work” or produce sound.