It all depends on the size of the glass - if we start from a point of everyones glass being half empty/full - for the rich (big glasses) to get richer, then that has to come from a lot of small glasses. Also, if every glass has to contribute say 25% of its contents, then in the small glasses it can make the difference between being thirsty, whereas in the case of th ebig glass, the 75% the glass owner would not notice this in terms of thirst as there is only so much water an individual can drink.

Moving away from analogies, money is the means of exchange for trading scarce resources. Wealth is that part of money taken out of the exchange system, so unless either there is a favourable balance of payments, or an increase in the money supply then retained wealth has to be at the expense of the poorer either in having less value for their income or a reduction in public spend.

Thirdly - wealth can only be relative. If we had 60,000,000 millionaires in the UK (200,000,000 in US?) then the purchasing power of the million would be reduced, i.e. nobody would be any better of as the value would be on paper only. Therefore wealth, or being well off, is dependant upon poverty.

Historically, this meant ownership of land, in the industrial revolution, ownership of the means of production.