- If an organism uses a process to convert something to something else, then that organism is a source of that something else through that process
- Photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves
- Hence, plants are a source of oxygen through photosynthesis

- Plants are a source of oxygen through photosynthesis
- A source of something increases the amount of that something
- Hence, plants increase the amount of oxygen

Therefore, plants increase the amount of oxygen
A scientist put several different plants in a sealed container. Every hour, she checked the oxygen in the container to see if it changed. How did the oxygen in the container most likely change?
- If an organism uses a process to convert something to something else, then that organism is a source of that something else through that process 
- Photosynthesis means green plants convert from carbon dioxide and water and solar energy into carbohydrates and food and oxygen for themselves 
- A source of something increases the amount of that something