system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Noise Canceling Headphones

They work great, but how do they cancel the noise?
gpt
Every sound is a wave in a medium. 

That means, sounds are vibrations in the air. Like a wave in your pool. 

Now, if you have ever been splashing in the pool, you may have noticed that two peaks of a wave, when they collide, will make a bigger wave. Also, a peak and a trough (the dip) , if they collide, they sort of cancel out. 

Well, if that peak is exactly the same shape, and exactly the same size as the dip (in reverse) the wave will totally cancel. 

What the noise cancelling headphones do is listen to the outside waves coming towards you, use a computer to calculate the inverse dips needed, and pump that into your headphones to cancel the noise.