"The reader may... wonder why when amber is rubbed with fur the electrons go from the fur to the amber, but when glass is rubbed with silk the electrons go from the glass to the silk? ...[W]e still don't know. The question involves the physics of surfaces of complex solids... In a purely empirical way, there has been developed... the triboelectric sequence... The electrification is most intense for objects... well separated in the... sequence. ...It is ironic that we still do not have a detailed understanding of frictional electrification, even though it was the first... to be studied... But... often... science progresses... by selecting problems that are as free as possible from irrelevant complications and... provide opportunities to get at fundamental principles..."