Why Non-Conductors Materials Get Charged ?

[blue_cristal]

If non-conductor materials like a rubber or glass rod have no free electrons then why they get charged with friction ? Is it due induction ( atomic polarization ) or something else ?

[Enthalpy]

Charges added or subtracted to this insulator.

 

Insulator means only that said charges will have difficulties to move.

[blue_cristal]

Thank you, Molecule.