I find reading the source of things can be incredibly helpful in some cases, but when I want to really grok something, I need to write code myself. When I'm just reading code it's easy to trick myself into thinking I understand something, but it's much harder to do that if I have to make a piece of code work correctly.

Peter Seibel wrote a great post on code reading, which hits on a similar point: http://www.gigamonkeys.com/code-reading/