When the user slide the middle bar, the light will appear. The lighting has two moods. When the rail is push towards the left, the upper light gradually show and the shadow is more energetic and active like a smiling eyelash or a rising feather while when the rail is going to the right side, the down light gradually appear and the shadow is more quiet and silent, like the eye closed or feather dropping.
The idea is inspired by the Chinese steelyard which contains two sets of units and it changes when users flip it. It is also related to the sensitive mood changes of the user when the scale is swing up and down.