At a certain point, as the years tick by, you start to realize that you have forgotten a lot of things. Typically, something you learned, some knowledge or skill you used to have, that you just haven't used for so long you no longer remember anything but that you once knew it.
I saw an article about electrical resistors the other day, and how the colored bands signify the rating. I used to know that. I used to be able to look at one and say what it was. Now all I remember is that I used to know it, and that resistors do something with, erm, electricity ... (right?)
But then I can still remember things that I "knew" at a much younger age and was never really of any significant interest, like the theme tune to "Champion The Wonder-horse".
Have a think - what have you forgotten you ever even knew?