Last Saturday my 11 year old step-sister used my iMac. She had seen the interface before, but never used it. I opened Safari for her, since I reckoned a browser is an easy piece of software that's the same on every OS, mostly. I left her alone after that (had to do the dishes).
After ten minutes I heard my iTunes playing and when I went to look at her screen I noticed that she had been browsing through my music and was writing a story in TextEditor. She showed me how she found out that she could minimise windows, enlarge them again, switch to another desktop (I use VirtueDesktop) and essentially where to find the software she needed in my dock.
Ten friggin' minutes. I mean... like... wow.