Physical interaction is the ability to change things in the physical world such as moving something and then a response is given by the object you are interacting with. It is like having a conversation with an object, but without words. Good physical interactions are those where you clearly understand what you can do, and what you are supposed to do to get certain responses, and those responses are given. Some examples of this are video games, motion sensor toilets, text to speech. Things that I would say aren’t interactive include things that aren’t dependent on human input to function. This means anything that you are able to put in data one time and run is not interactive. To be interactive it has to be looking and changing based on your input at all times. I agree that for the most part interactivity is hard to achieve because people don’t understand it enough to create new things as Crawford said, however I think this is also not done easily because it take a great amount of creativity and foresight, which are rare. Not to say that this can’t be achieved, but the most fortuitous implementation of physical interactivity is developed over long periods of time, making progress seem slower than it really is.