- A watch requires electrical energy to operate
- If something is required then that something must be provided
- Hence, a watch must be provided with electrical energy

- A watch must be provided with electrical energy
- Electrical energy is a kind of energy
- So, a watch must be provided with energy

- A watch must be provided with energy
- A source of something emits / produces / generates / provides that something
- Hence, 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
- So, 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 cells convert solar energy into electrical energy
- Therefore, solar cells are a source of electrical energy

- Solar cells are a source of electrical energy
- Electrical energy is a kind of energy
- So, solar cells are a source of energy

- A watch must be provided with an energy source
- Solar cells are a source of energy
- So, solar cells provide an energy source for a watch

Therefore, solar cells provide an energy source for a watch
A student examines a new watch design. The watch has a small solar cell on its face. The main function of the solar cell is to
- A watch requires electrical energy to operate 
- If something is required then that something must be provided 
- Electrical energy is a kind of energy 
- Solar cells convert solar energy into electrical energy 
- If an object converts something into something else then that object is a source of that something else 
- A source of something emits / produces / generates / provides that something 
- A cell is a kind of object