 Hendry (2006) points to a potentially more difficult problem, which is that any liquid sample of water, however pure, contains not only H2O molecules but also H3O+ and OH− ions. These ions cannot be regarded as impurities, since they are also an inevitable consequence of the polar structure of the H2O molecule. Hendry’s solution is to regard H2O molecules as the constituents of water in rather the same sense that eggs and flour are the constituents of a cake—they go into making the cake but need not retain exactly the form they started with. Water is made from bodies of H2O molecules, but not all those retain precisely that structure.