Yip, you were almost right there haha it's an extra neutron the H has (so it's an atom of Hydrogen with 1 Proton and 1 Neutron), converting it to Hydrogen's isotopic form - Deuterium.like normal water and heavy water (not Hard water, thats completely different), both are H2O, but hydrogen has an extra proton in its nucleus in heavy water
Heavy water isn't particularly naturally occuring, and you certainly couldn't just turn normal water into heavy water with the power of thought, or words written on a piece of paper wrapped around a jar... Since you would have to create mass to make the new neutron. Speculating beyond that is pointless, since you couldn't do that in the first place haha but needless to say.. Even if you could create neutrons, you'd have trouble forcing them to 'bond' with the protons in the Hydrogen atoms...
Blech, sorry, I'm getting all scientific again haha. Ignore me, I don't mean to be an ass, it's just my scientific side showing itself..