There are many different operating modes for an AFM. One is the "contact mode", where the tip is simply moved across the surface and the cantilever deflections are measured. Another mode is called "tapping mode", because the tip is tapped against the surface as it travels along. By controlling how hard the tip is tapped, the AFM can move away from the surface when the needle feels a ridge, so that it will not hit against the surface when it moves across. This mode is also useful for biological samples, because it is less likely to damage a soft surface.