Gypsum board, also known as drywall, is fire-resistant due to its core of chemically combined water (calcium sulfate dihydrate), which helps slow the spread of fire. When exposed to high heat, the water is gradually released as steam, helping to cool the surrounding area and delay combustion. Fire-resistant gypsum boards often contain additional additives and fiberglass reinforcement to enhance performance and are commonly used in fire-rated wall and ceiling assemblies in residential and commercial buildings.