Here's why:

* Chemical Equation: 2 H₂O₂ → 2 H₂O + O₂

* Energy Change: The reaction releases energy, which is evident in the form of heat and sometimes light (if the reaction is vigorous).

* Enthalpy Change: The enthalpy change (ΔH) for this reaction is negative, indicating that the products (water and oxygen) have lower energy than the reactants (hydrogen peroxide).

In simpler terms: When hydrogen peroxide breaks down, it releases energy, making the surroundings warmer. This is why you might see a slight increase in temperature if you observe the decomposition of hydrogen peroxide.