Their metaphor for materials is the combination of two ideas: A streamlining/optimization of screen area, and a set of rules that define how things move. The

Their metaphor for materials is the combination of two ideas: A streamlining/optimization of screen area, and a set of rules that define how things move. The material metaphor is based on things we can touch in real life. However, this metaphor is not going to be strictly adhered to in order to take advantage of the technology it's implemented on and provide a good user experience.