Why a knife sharpened on rotating wheel produces sparks?

When a knife is sharpened on a rotating wheel, the friction between the wheel and the knife blade generates a lot of heat. This heat causes the metal of the knife blade to vaporize, and the tiny particles of metal that are vaporized then ignite in the air, producing sparks.

The sparks are also caused by the fact that the rotating wheel is constantly removing material from the knife blade. This removal of material creates tiny pieces of metal that are then heated by the friction and ignited.

The sparks that are produced when a knife is sharpened on a rotating wheel are not harmful, but they can be a nuisance. If you are sharpening a knife in your home, it is a good idea to wear eye protection to protect your eyes from the sparks.