Concepts that take a long time to get

In my life I have on occasions found concepts that seemed hard to understand.
In retrospect I think that I had not assimilated the background to prepare myself, but whatever the reason
I'll put commit some of these of these to my blog, because I know that it is going to be hard to ask other people for such concepts. Teachers are the ones who have the biggest collection of concepts that are hard to get because they meet these in their students all the time.

One concept I found very hard to figure out was the notion of datum in geography.

Another concept I had trouble was with Universal Element in Birkhoff and Maclane's book on Algebra.
It was very hard to remember even if the definition was very short, whats more it seems to mix ideas
from different levels that should not be there: an element of a set and category theoretic ideas which I thought should have existed at another level.

The various kinds of joins in SQL were rather hard to get at first. I felt I should "get back to those later".


I also remember a friend having a lot of difficulty explaining what he meant by the "time value of money".
I think it was also because I refused to accept his conceptual starting point.



I do know that a lot of people have trouble with pointers in the C language.

I have been told that students have many problems with the notion of limits in Calculus. I never had problems here.

I have seen peole have huge problems with how loops could work in computer "functional languages".