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The regular ice cubes float in water. The deuterated ice cubes sink.
Water is less dense in its solid state than in its liquid state. This property allows solid water (ice) to float in liquid water (objects less dense than the liquid they are in float while objects more dense than the liquid sink).
Ice made from deuterated water, on the other hand, doesn't float. The hydrogen atoms in a deuterated water molecule are replaced with deuterium atoms. Deuterium is an isotope of hydrogen that is twice as heavy due to an added neutron. Deuterated ice is about 10% heavier (and, therefore, more dense, because the water molecules still take up the same space) than regular ice. The density of deuterated ice turns out to be 1.105 g/cm3. This density is greater than the density of liquid water, so the deuterated cubes sink.