A property of polymers is their glass transition temperature, T_g, which is the approximate temperature below which the polymer is hard and rigid, but above which the polymer is soft and flexible. The T_g of polyethylene is a chilly -125°C, which is why polyethylene food wrap is flexible at ambient temperatures. Consider the two polymers polyethylene terephthalate (PETE) and polystyrene (PS). Which do you suppose has the higher T_g? Dip some plastics of these two polymers in boiling water to find out. A common polymer used to make chewing gum is polyvinyl acetate, with a T_g of about 28°C, which is below body temperature but above room temperature. That’s why most chewing gums are hard until they soften up in your warm mouth. Drink ice water while chewing gum and note how it quickly hardens.