system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do noise cancelling headphones work?
gpt
They contain microphones that "listen" to the environment, and send a signal to the speakers in the headphones that are 180 degrees out of phase with those sounds. That is, they are the "opposite" of each other. If they do that quickly enough, and at the right volume level, the two sounds mostly cancel out.  This doesn't work all that well for some sounds (like human speech), but for things like the hum of airplane engines it can do a pretty good job.