A man sees a lemonade stand

A man sees a lemonade stand run by a small girl. He notices that 1 glass of lemonade is $1 and that 3 glasses are $5. The man explains to the girl that buying 3 glasses for $5 is more expensive than buying 3 glasses for $1 each. The girl disagrees and says that it costs less to buy 3 glasses for $5. He tries to explain once again, but the girl still says that buying 3 glasses for $5 is cheaper. Frustrated, the man buys 3 glasses of lemonade for $5 and 3 glasses of lemonade each for $1. He shows the girl that he bought the same amount of lemonade for $3 instead of $5 and states that the girl is wrong. The girl then replies, "I may be wrong, but you're the one who just bought 6 glasses of my lemonade!"