- Cellular respiration is when a cell converts from oxygen and carbohydrates into carbon dioxide, water, and energy
- Living things require respiration to use energy
- Hence, living things must convert oxygen into carbon dioxide to use energy

- Living things must convert oxygen into carbon dioxide to use energy
- Breathing is when animals convert oxygen into carbon dioxide
- So, living things must breathe to use energy

- Breathing out is when animals exhale carbon dioxide from their lungs into the air
- Animals are a source of carbon dioxide by exhaling
- Hence, animals are a source of carbon dioxide by breathing

- Animals are a source of carbon dioxide by breathing
- A source of something produces that something
- Hence, animals produce carbon dioxide by breathing

- Living things must breathe to use energy
- Animals produce carbon dioxide by breathing
- Therefore, animals produce carbon dioxide by using energy

Hence, animals produce carbon dioxide by using energy
Question: When animals use energy, they produce
Context: - Breathing out is when animals exhale carbon dioxide from their lungs into the air 
- Animals are a source of carbon dioxide by exhaling 
- Breathing is when animals convert oxygen into carbon dioxide 
- A source of something produces that something 
- Cellular respiration is when a cell converts from oxygen and carbohydrates into carbon dioxide, water, and energy 
- Living things require respiration to use energy