Over a period of time, it seems like a layer of dust covers your deep understanding and knowledge of a subject matter, where in once I was considered an expert. My knowledge is rusting and it's time to re-read some old classics and books I used to learn the subject matter from.
I've been trying to learn C#, Python, etc, but I realize that I need to revisit my C skills and my C99 knowledge base again. In summary, knowledge rusts over a period of time, specially the items not practiced very often.
Get ready to clear away the dust from your rusting areas as well, as I explore well known areas and newer areas with a "I used to know, but forgot" perspective.