When a glass rod is rubbed with a silk cloth, the glass rod becomes positively charged while the silk cloth acquires a negative charge. This happens because rubbing the glass rod with the silk cloth causes the glass to lose electrons quickly, and the silk absorbs electrons from the glass rod. As a result, the glass rod becomes positively charged, while the silk cloth becomes negatively charged. This phenomenon is due to the triboelectric effect, which is the transfer of electrons between two materials that are in contact and then separated.