When the cutting edge of a knife is put against a fast rotating stone to sharpen it, sparks are seen to fly because:

⇒ The friction increases between the edge of knife and rotating stone.

⇒ As a result, it increases the temperature of the knife and stone

⇒ Friction produces heat.

⇒ As the friction is very large in this case, thus the temperature increases to such a level that the sparks are produced.