A disposable paper cup can be waterproof due to a special lining or coating applied to its interior surface. This lining is typically made from materials like polyethylene (PE) or polylactic acid (PLA), which are inherently water-resistant or can be treated to repel liquids effectively. The coating prevents the paper fibers from absorbing moisture, thereby keeping the cup waterproof and maintaining its structural integrity even when filled with hot or cold beverages. This design ensures that the cup remains reliable and functional throughout its use, providing a convenient solution for beverage consumption on the go.