system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do people who speak in sign language also move their mouth when speaking?
gpt
For the same reason people who speak with words move their hands. It adds context and adds clarity to the communication.

Many deaf people also learn to read lips (because only a minority of people know sign language). Many deaf people don't know sign language as well as you'd think! Especially those who lost their hearing later in life.

The combination of gestures and lip movement helps them understand what is being transmitted.