Question

A spark is produced when two stones strike each other. why?

Answer 1

When two stones are stricken together there is friction between the stone which causes heat and ultimately spark.

Answer 2

When two stones are rubbed a frictional force persists among them . Due to rubbing displacement would be there. As both force and displacement are present so some work would also be there (except zero work case ). As work is done on two harsh bodies (rocks) so energy would be produced . In this case it is heat energy. Therefore a spark is produced.