Bombard an ordinaryhydrogen atom with microwaves of the right frequency and you will lift it out of its ground state by flipping the spin of its electron.
Heavy water is heavy because the hydrogen in it weighs twice as much as ordinaryhydrogen (it has a proton and a neutron in its nucleus, instead of just a proton).
Never mind that they showed the difference between heavy water (with deuterium atoms) and ordinary water (with hydrogen atoms) in a way that no image had done before.