I remember when I was a kid, I thought it would be awesome to be a spy. I could have all kinds of cool gadgets to play with. The other day, Lifehacker had a story about building a secret knock door lock.

Basically the idea is you program in the knock you want. The lock listens to the knocks on the door. When it detects an accepted knock pattern it unlocks the door. If it doesn’t detect the accepted lock, it doesn’t unlock it.

You can read the full story by Steve Hoefer. Below is the video.