- Humans discarding waste in an environment causes harm to that environment
- Discarding is similar to throwing
- Garbage means waste
- Hence, humans throwing garbage in an environment causes harm to that environment

- A body of water is a kind of environment
- A stream is a kind of moving body of water
- Hence, a stream is a kind of environment

- Humans throwing garbage in an environment causes harm to that environment
- A stream is a kind of environment
- So, humans throwing garbage into a stream causes harm to the stream

Therefore, humans throwing garbage into a stream causes harm to the stream
Which human activity often has a harmful effect on the environment?
- A body of water is a kind of environment 
- Humans discarding waste in an environment causes harm to that environment 
- Discarding is similar to throwing 
- A stream is a kind of moving body of water 
- Garbage means waste