Eighteen kinds, and no more?

   Actually, the varieties of water may be much more numerous. Besides the natural isotopes, there are also man-made radioactive isotopes of oxygen: oxygen-14, oxygen-15, oxygen-19, and oxygen-20. And just recently the number of hydrogen isotopes has also increased: we can now speak of H⁴ and H⁵. 

   Now if we take into account the man-made isotopes of hydrogen and oxygen, the list of possible waters increases to over 100. You can easily count up the exact number yourself. . .

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