When people perceive the world, what they see is based on the physics of light reflecting off surfaces and entering their eyes. Their brain then processes the raw data so that photoreceptor activity becomes perceptual awareness. Most textbooks and chapters on sensation and perception follow this formula, building student understanding of perception as progressing from the raw data of light to the biological response of photoreceptors to more complex processing of edges and objects in the brain. This approach is often called bottom-up processing. Top-down processing, in contrast, occurs when people's expectations, emotions, and bodies affect how they see the world. In this article, I review diverse evidence suggesting that perception is not solely a result of bottom-up processing. I also suggest ways that we should inform students of this complexity.