Here's why:

* Cellular Respiration: Mitochondria are the powerhouses of the cell, responsible for cellular respiration. This process breaks down glucose (sugar) to produce energy in the form of ATP.

* Electron Transport Chain: A key step in cellular respiration is the electron transport chain, which occurs within the mitochondria. This chain of reactions involves the transfer of electrons and the pumping of protons across a membrane.

* Water Formation: The final step in the electron transport chain involves the use of electrons and protons to combine with oxygen, forming water (H₂O).

Therefore, water is a by-product of the energy production process in mitochondria.