Daniel put on his noise cancelling headphones on a plane and the engine noise just disappeared.

He assumed it was thick padding blocking the sound.

He was wrong -- in a really interesting way.

Noise cancelling headphones do not block sound. They create more sound. And that new sound cancels the old sound out. You fight sound with more sound.

Here is how it works.

Sound travels in waves -- peaks and troughs repeating over and over. If you take two identical waves and line them up so peaks match peaks, the sound gets louder. But if you create an exact opposite version -- peaks matching troughs -- the two waves meet and the sound becomes much quieter. When the timing and amplitude line up very closely, they can come very close to cancelling it out entirely. This is called destructive interference.

Noise cancelling ...