Charged by rubbing

Different materials have different degree of tendency to attract or lose electrons. When two materials are rubbed, the one with greater tendency to attract electrons will acquire a negative charge, whereas the other material will acquire a positive charge because of loss of electrons.

The tendency for a material to gain or lose electrons depends on its molecular structure (i.e. the strength of electrostatic force that binds the electrons to the nucleus).

Materials with different tendency to attract electrons have been arranged in a series, the triboelectric series, with ascending order of materials that tends to gain electrons (i.e. those on top of the list have the least tendency to gain electrons, in other words, they have strong tendency to lose electrons and become +ve charged).

Because glass is higher than silk in the triboelectric series, silk has a greater tendency to gain electrons than glass. As a reult of rubbing, electrons move from glass to silk. This results in glass becoming +ve charged and silk becoming -ve charged.

You could refer to the following web-page on the triboelectric series,

http://www.electronicsteacher.com/succeed-in-physical-science/static-electricity/the-triboelectric-series-of-materials.php