Flammability
Flammability is how easily a material catches fire and how fast it burns.
Think of it this way
Dry paper and wet paper sit side by side on a kitchen counter. The dry paper catches fire fast when held to a flame. The wet paper barely burns at all. Flammability works like that: the drier and looser a material is, the faster it catches fire.
Explaining flammability by grade level
Some fabrics catch fire fast. Others are slow to burn. Cotton burns quickly. Baby clothes are made to resist flames and stay safe longer.