Illusions are experiences of perception in which one perceives stimuli incorrectly, not completely, or in distorted way. In such cases the illusion is unrelated to the human sensory mechanism. Some are caused by external factors (unrelated to the receiver's sensory organs and brain), as in the case of the “Doppler effect”, when one hears, say, the siren of an ambulance change its pitch as it passes by when, in reality, the pitch stays the same. The ears and brain of the receiver function normally, but this function is impeded by external natural factors like distance, interference of elements in the air, wind and its direction, etc. Illusions of this kind are called stimulus-distortion illusions. Other illusions are more receiver-related, and are referred to as perceiver-distortion illusions. These are caused by the way in which the human brain functions and, in particular, its tendency to group similar objects together as well as to “complete” unfinished objects. The latter function is what makes watching a movie possible, since it “blocks” perception of the intervals between projected stills and makes them look like a “moving” picture.