Knock Knock are pairs of networked woodpeckers.
When you physically make one woodpecker knock on the wall, its connected pair, wherever it is located, automagically knocks as well. This simple and playful action and reaction allows people to develop and improvise - not to mention interpret - their own ways of communicating.
A longer description can be found on the CIID website, and I also wrote a long post on this project and the process on my blog. This project was created with Shruti Ramiah as part of a four week course at CIID on Tangible User Interfaces.