Homer entered a bar.
-- What beers do you serve here?
-- Guinness at $3 a can, and Duff at $5, - answered the barman.
Homer feverishly searched his pocket, scraping together $3 in small coins. He passed the money to the barman and took a can of Guinness in return. Right before opening it, he started to wonder: "Wait a minute! I just gave $3 to the barman and I am holding a $3 beer can here. If I give this can to the barman now, I will have given $6-worth of stuff to him in total! Consequently, he will owe me a bottle of Duff and $1 in change back! I'm a genious!"
