Think about the ways that mobile technologies affect interaction in public and social spaces. Spend one full hour in a subway observing people on their mobile devices. Take notes on what you observe. Where possible, capture images for your documentation, but don't cross a line with people's privacy. Refer to the Rodger's chapter on Observing Technology in Use listed in Module 1.
Develop a concept for a technology that makes interactions on the subway more interesting. It might be about how people interact with each other (if that really happens) or maybe how people interact with the MTA or advertising... No limits here except your creativity. Describe the problem/opportunity, explain the apps benefits, how it works/functionality/user experience, what it looks like. Provide a rationale for your design choices. Include one or two sketches/images of how the application works. I'd encourage you to work in pairs.