The phi phenomemon or principle of apparent motion refers to the illusion that an object is moving in space, when, in reality it is stationary. The illusion is created by presenting a series of stationary objects in rapid succession, each of which has a slightly different position from the previous object. This principle underlies the making of movies and animations. Scroll down to see the complete animation below which should be a familiar one. It is often seen on highway road signs to direct drivers to change lanes.