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Lennie thinks very much as a child, He lives in the moment; he is thirsty so he drinks. He drinks as much as he can. Where George is cautious about drinking stagnant water, Lennie drinks with abandon, George warns Lennie, "You never oughta drink water when it ain't runnin', Lennie."
Also George is very much in parent mode. For George, this is pretty much all the time.