For a period of time, Energizer batteries used mock commercials for seemingly boring, mundane products. A few seconds into the mock commercial, viewers heard the distinctive drumbeat of the Energizer Bunny before it marched across the screen. Several viewers said that when they heard the drums, they looked at and attended to the commercial because they liked the bunny. However, over time, viewers stopped paying attention to the commercial. What theory suggests that if a stimulus doesn't change, over time we adapt or habituate to it and begin to notice it less?