premise: a watch requires electrical energy to operate sent2: if something is required then that something must be provided sent3: electrical energy is a kind of energy sent4: solar cells convert solar energy into electrical energy sent5: if an object converts something into something else then that object is a source of that something else sent6: a source of something emits / produces / generates / provides that something sent7: a cell is a kind of object.
hypothesis: solar cells provide an energy source for a watch