I am engaged in the plastic industry. The bottom of a safe plastic cup should be marked PP, or a triangle with an arrow. The interior marked with 5 indicates that the melting point of PP..PP is as high as 167℃, and many plastic products of microwave ovens are made of this material, so PP is the only safe plastic after heating. Other plastics are toxic when heated. There is no sign on the bottom of the glass, because the main components of glass are silicon dioxide and silicate, which are non-toxic substances. Just to prevent the glass from cracking or bursting due to uneven heating and concentrated internal stress.