Final answer:
Friction causes objects to become charged through the transfer of electrons between them when rubbed together, which can result in one object becoming positively charged and the other negatively charged.
Step-by-step explanation:
When two objects are rubbed together, friction causes the objects to become charged because electrons are transferred between them. During the process of rubbing, if one material has a greater affinity for electrons than the other, a transfer of electrons can occur, causing the object that loses electrons to become positively charged while the object gaining electrons becomes negatively charged. This phenomenon, which is a fundamental concept in static electricity, demonstrates that charging can occur by contact or rubbing and is not due to the loss of neutrons or protons, nor is it an electric discharge during the friction process.