In this movie the sister of Hero report to her mother what she saw saying "My little brother was spanking the monkey". Wondering you can ever guess what her mother reply. Her mother reply "What? Did he spank the daughter?" Because
the expression 'spank the monkey' translated as " hit the daughter" in South Korea.(ddal=daughter chida=hit, spank)
When the protagonist was masturbating and spot by his sister, he make a frown face and matched to the crying sound of the baby with excellent timing.