I came across a phrase the other day. If you cannot explain it to a five year old, you don't really understand it :-) I thought it very true and I am pretty sure some of the teachers on here will agree. My list, early on, included UNIX. I did not realise how much I did not know until I taught a beginners course. For any serious students of anything I would recommend teaching it to others. You will come on in leaps and bounds!