As everybody knows, two representative multi-touch gestures are multi touch pinching and rotation. I'd like to ask you a seemingly simple question. What do you think the difference between those two?
Answer 1: the first is for changing the size of a digital object while the latter indicates to rotate it. And that's all.
However, when you think a little bit deeper, there is another difference between them from the perspective of interaction.
When you apply the pinch gesture to a real photo, nothing happens. The photo just remains there without any physical changes. However, you can physically rotate a real object by applying a rotation gesture.
Answer 2: The pinch gesture is a metaphored interaction, while the rotation gestures is the exact simulation of the real world.
PS 1: So, we have two types of (multi touch) gesures: type I and type II. Type I gestures are truly natural that reflect physics in our real world. Typical examples include rotation and panning. Type II, on the contrary, need some active interpretation by the object of the interaction. The pinch gesutre on the touch screen and waving someone to come closer in the real world might be categorized into type II.
PS2: Thanks JoshB for a helpful comment.