system
Given the following post, respond as if you're commenting on Reddit:
human
When a sound-wave travels in an open pipe, what reflects it back in order to form a standing wave?

I understand that in a closed pipe, the closed end reflects the sound-wave back and forms a standing wave, but there's no physical thing to reflect off in an open pipe. So how does it reflect?
gpt
When inside, the sound wave is exerting a pressure on the inner wall of the pipe. As the compression wave reaches the open air, that pressure causes the air particles to overshoot the end of the pipe sightly. This creates a low pressure area inside the pipe which sucks air back in, creating another wave that propogates back in the opposite direction.  
The resultant wave is considered to originate slightly beyond the end of the pipe.