I believe that Interaction Design is the practice of designing interactive digital products, environments, systems, and services. It is mainly the creative process and thoughts of people. Just like many other design fields, interaction design also has an interest in form, however its main focus is mainly on behavior. It is imagining things as they might be rather than focusing on how things are and work. That explains the difference between interaction design as being a design field rather than a science or engineering field. Some challenges I believe would be to know exactly what your mission is. You must come up with something that is user friendly and easy to communicate to that user. You must figure out what exactly you should provide for the user and how it should work and feel. I believe that interaction design comes from science and engineering as I mentioned earlier. There is a reason why this is a design field rather than a science or engineering, however I believe it draws a lot of knowledge from those fields as well.