Your question has to do with the Triboelectric Series. "Tribo" means to rub so the series has to do with materials that acquire an electrical charge by rubbing. Some materials tend to give up electrons while others tend to gain electrons. The Triboelectric Series ranks materials according to their tendency to gain or lose electrons. Glass is a material that becomes highly positively charged when rubbed with a cloth. This means that it gives up electrons (loses negative charges.) Teflon is a material that becomes highly negatively charged when rubbed with a cloth, meaning it gains electrons (gains negative charges).