Pour some drain cleaner into a glass and immerse a paper cup in it. After an hour, take the cup out and use a plastic stick to clean off the yellowed paper and reveal… plastic!
The sodium hydroxide in the drain cleaner destroys the paper’s structure, making it easy to clean away. This reveals the plastic film under it, which prevents the paper from getting wet and the beverage from leaking. The film is made of polyethylene, so this "paper" cup can not be recycled with regular paper. It requires separate collection and special processing.