Here's why:

* Exothermic reactions release energy into the surroundings, usually as heat.

* Combustion is a chemical reaction where a substance reacts rapidly with an oxidant, typically oxygen, to produce heat and light.

The combustion of methane (CH4) with oxygen (O2) produces carbon dioxide (CO2) and water (H2O) along with heat and light:

CH4 + 2O2 → CO2 + 2H2O + heat + light

Here's the breakdown of why this reaction is exothermic:

* Bond breaking: Breaking the bonds in methane and oxygen requires energy.

* Bond formation: Forming the bonds in carbon dioxide and water releases energy.

* Energy balance: The energy released during bond formation in the products is greater than the energy required to break the bonds in the reactants. This difference in energy is released as heat, making the reaction exothermic.

Therefore, methane itself is not exothermic. The exothermic reaction is the combustion of methane, where the energy released by forming new bonds is greater than the energy required to break existing bonds.