In order for us to perceive something, there must be a source that triggers our perception. That source is called a stimulus. A stimulus is composed of some kind of energy that our senses are tuned to detect. For example, the visual system can detect electromagnetic energy, that is, light, and the auditory system can detect vibrational energy, that is, sounds. In turn, the sense of smell, called olfaction, is tuned to detect chemical energy stored in some particular compounds called odorants. The odorants constitute the olfactory stimuli. This entry covers units of change, management of the olfactory stimulus, complexity of the olfactory stimulus, and sniffing.