Sensors are often a requisite for a successful end product when making a new interface for human-machine interaction. Contact microphones are one of my favourite sensors, and now we have a new batch of them in the workshop.
Contact mics are designed to pick up sound transmitted through hard materials: A contact mic glued to a table will pick up sounds that are made when things hit the table, but not what those around the table talk about.
Glue it on your floor, and make your computer aware of feet, toys, parts hitting your floor. By using a grid of contact mics you can even get a rough approximation on the position of things that are moving around!