Usually capacitive touchscreens sense a capacitance difference, which is set by the dielectric permittivity (ε) of the material above. Ours is close to water's,

Usually capacitive touchscreens sense a capacitance difference, which is set by the dielectric permittivity (ε) of the material above. Ours is close to water's, but an insulating layer from another material will yield different measurements. I am guessing that touchpads use similar tech. A fun fact is that capacitive sensors such as the one used on touchscreens are fully 3D-capable, but that seems to be rarely used (I've seen it once or twice on Samsung phones, and Sony's "glove mode").