According to the legend, a person who goes to the third stall in the girls' bathroom on the third floor and knocks three times before asking "are you there, Hanako-san" you will hear a voice answer, "I'm here."
Hanako-san is a popular and widespread urban legend, often played by school children as a rite of courage, or a method of hazing for new students, similar to the Bloody Mary urban legend in Western schools.