Scent

https://carsinspections.com/transmission/when-to-change-car-automatic-transmission-fluid/

The idea of running a car engine on water is not about directly using water as a fuel source in the same way gasoline is used. Instead, the concept relies on breaking down water into its constituent elements: hydrogen and oxygen. Hydrogen, a highly combustible gas, can then be used as fuel to power an internal combustion engine or a fuel cell.