A man walks to a bar. Next to him at the counter is a woman. The night is busy - he waits for a long time until another man, the bartender, turns to face him, gesturing with his hand for an order.
The man orders an ice water - but the woman has been waiting even longer, so he decides to order for her as well. As it happens, she wants two ice waters. "Three ice waters?", the barkeep says, incredulous and irked. "That'll be three fifty", he says, making up the price on the spot. The woman pays, the man can never pay her back. For all time, a monetary imbalance will exist between them - an Eternal Thing.