What is the key characteristic of flammable solids?

Flammable solids are materials that can easily ignite and sustain combustion. The key characteristic of such materials is their ability to catch fire readily when exposed to an ignition source, such as heat, sparks, or open flames. This property makes them hazardous during various operations, as they can ignite quickly and potentially lead to fires.

In contrast, while some flammable solids may produce toxic fumes when burned, this is not a defining characteristic for all flammable solids. Similarly, while certain flammable solids may not react violently with water, this alone does not make them a flammable solid. Overall, the essence of flammable solids lies primarily in their propensity to easily catch fire, posing a significant fire risk in various contexts.