My one year old granddaughter teaches gramps about user interface (UI)
You cannot test for usability if you are familiar with the interface. Yesterday my 18 month-old granddaughter had her first corn on the cob.
It was very slow and frustrating for her. For her, this was corn with an all new UI. My daughter-in-law took bites off the cob in an effort to show her. She put the cob in the tyke's mouth. For her, this was a whole new kind of corn. It was corn on Linux. I realized that UI is an issue that we struggle with from childhood and to be able to provide good usability we have to really be at the user level. We cannot assume that users will relate to an interface because it is comfortable for us. We can only test if we test with real users who view the interface from a non-user perspective.