If you're looking to make your alcohol weaker, adding water is a good way to do it. When water is added to alcohol, the two substances mix and form a new solution with a lower concentration of alcohol. This is because the water molecules take up space in the solution that was previously occupied by ethanol molecules, thus reducing the overall concentration of ethanol. So if you're looking to cut down on the alcoholic content of your drink, adding a splash of water can help.