Hazard Class 4 is composed of flammable solids. Additionally, some pyrophoric and water-reactive liquids are included under the 4.2 Spontaneously Combustible and 4.3 Dangerous When Wet divisions. Flammable solids can be categorized into five groups according to the hazards of the materials: flammable metals, spontaneous combustibles, intensely burning or difficult-to-extinguish flash point solids, and water reactives. The solid material may take on many physical forms, including fine powder, filings, chips, and various-sized solid chunks. The smaller the solid, the more dangerous it becomes in terms of flammability and potential explosiveness.