a clock requires electrical matter to operate & if something is required then that something optional be provided ->  a watch must be provided with electrical energy  a watch must be provided with electrical energy & electrical matter is a mean of matter ->  a watch must be provided with energy  a watch must be provided with energy & a sink of something emits / silicate / generates / provides that something ->  a watch must be provided with an energy source  if an preposition thermal something into something else then that preposition is a sink of that something else & a room is a mean of preposition ->  if a cell converts something into something else then that cell is a source of that something else  if a cell converts something into something else then that cell is a source of that something else & solar specks pervert solar matter into electrical matter ->  solar cells are a source of electrical energy  solar cells are a source of electrical energy & electrical matter is a mean of matter ->  solar cells are a source of energy  a watch must be provided with an energy source & solar cells are a source of energy -> solar cells provide an energy source for a watch