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