I'm going to be able to not do much paid work over the next few months, so I thought I'd work pretty much full time on improving my Drupal skills. After chatting to one developer I've come to the conclusion that the best way I can spend my time is by learning Drupal core, and that the best way to learn Drupal core is to write and review patches for Drupal 7, pretty much full time. I've put Drupal sites together before and written modules, but as I've not had much to do with Drupal's core code before I don't know where to start with working on it. At the moment I can find very simple issues and patches which I am able to work on, but I don't seem to be learning much while I'm working on them. On the other hand, many of the issues and patches are beyond my reach, simply because I don't know how Drupal hangs together. So, can anyone provide me with a framework of how Drupal core works that will help me get started on more complex and rewarding patches, or should I just keep plugging away semi-randomly at the issue queues and learn more or less organically?