There are three types of stimulus classes: formal, temporal and functional. Stimuli in a formal class share physical features with one another. In other words, the topography of the stimuli are similar. Some examples of formal similarities are size, shape, weight, color, magnitude, intensity, and general appearance. Stimuli in a temporal class exist in a similar temporal space as one another. In other words, the temporal class refers to time in relation to the behavior of interest. There are antecedent temporal stimuli and consequence temporal stimuli. Antecedent temporal stimuli have a similar evocative or abative effect as one another such as an instruction or a signal. Consequence temporal stimuli appear after a certain response or behavior like a certain item or toy. Finally, stimuli can exist in a functional class. Stimuli in a functional stimulus class are commonly identified through a functional analysis. The stimuli in this class share a similar effect on behavior. For example, when you hear music, you start to dance, regardless of what type of music you hear.