The hydrogenation of nitrous oxide by PNP pincer ruthenium complexes supposes a promising way to functionalize a hazardous gas and reduce the greenhouse effect, generating dinitrogen and water. Here, by DFT calculations we describe not only the whole mechanism for such a green transformation but we unravel the fundamental role of water, without which the reaction could not go forward. Water assists mandatorily in the H transfer to generate the hydroxyl group together with the release of dinitrogen.