Some thoughts that I had. Not specific to any language. But if the answers depends on the language, I know c++, java and actionscript.
1- When you declare an integer: "int i = 0", what is int? Is it a class? How does it work inside? Is there a source code for how the int works?
2- The same doubt about structures: if, else, for, etc (I think we call them structures). What are they? How do they work? Is there a source code?
3- Is it possible to make a pointer point to a specific memory address?
int * p;
p = 0x00FF32
Is it possible somehow?
4- Is it possible to know what type of variable the memory address 0x00FF32 is containing? What other info I can get about this address?