Hydrogen moderators include water (H 2 O), heavy water (D 2 O), and zirconium hydride (ZrH 2), all of which work because a hydrogen nucleus has nearly the same mass as a free neutron: neutron-H 2 O or neutron-ZrH 2 impacts excite rotational modes of the molecules (spinning them around). Deuterium nuclei (in heavy water) absorb kinetic energy less well than do light hydrogen nuclei, but they are much less likely to absorb the impacting neutron. Water or heavy water have the advantage of being transparent liquids, so that, in addition to shielding and moderating a reactor core, they permit direct viewing of the core in operation and can also serve as a working fluid for heat transfer.