"What don't I know?"
I focus on client/server/user-oriented methods of design and development at this point, with no other subjects intruding. It's a great way to start your day, and gets your brain thinking in a problem solving manner.
I wake up, do my morning stuff, make some coffee, and quickly think about the previous day with that exact question. As a developer, it shouldn't take much time to cherry pick one from your "personal backlog of issues".
Then, I drink my coffee and research it for approx. 10 mins. If it's something important or interesting, it'll stick like a mental bookmark, to continue learning about whatever it is.