My husband and I recently took an Alaskan cruise. As we settled ourselves for sailaway in front of large windows on one of the highest decks, I heard a little girl ask, "Did we start moving yet? How will we know when we're moving?" I don't know what her mother said, but I do know the answer: you fix your gaze on what isn't moving.
I was looking at the building in this picture I took; when the ship starting pushing away from the pier, I knew we were moving because of our view through the window in relation to the stationary building.