So, after several months of using your iPhone exclusively, you have occasion to use another phone briefly for whatever reason. In my case, I still have my Motorola Razr, which is still in service with U.S. Cellular (at first I wanted to keep it in service as a backup phone, but now after almost 5 months, I just haven't gotten around to cancelling my 2-year contract on it which still has over a year to go..haha).
So, you pick up the other phone, you see icons on the screen, and you touch them, expecting something to happen. You may even touch or push them TWICE, still not knowing why something isn't happening.
Then, it dawns on you, "Oh, this isn't my iPhone. I have to push a button somewhere." 