Flammability is a chemical property that tells whether a substance reacts in the presence of oxygen (O2).

Here's why:

* Combustion: Flammability is essentially the ability of a substance to burn. Burning is a chemical reaction called combustion, which requires oxygen as a reactant.

* Exothermic Reaction: Combustion is an exothermic reaction, meaning it releases heat and light energy.

* Fuel and Oxidizer: The substance that burns (like wood, gasoline, or paper) is called the fuel. Oxygen acts as the oxidizer, which combines with the fuel to produce the combustion reaction.

So, a flammable substance is one that readily reacts with oxygen to produce a fire or flame.