What does it mean for girls to sing in the shower?

It is said that 80% girls masturbate in the shower and 20% girls sing in the shower. When a boy heard this, he went back to school and asked a female classmate, "Do you know what the 20% girls are singing?" Female classmate: "I don't know." The boy smiled and walked away, and the people who heard him also laughed. Seeing that everyone was laughing, the female classmate shouted, "I really don't know ~ what's so funny!" " "At this point, the laughter is bigger ...

She replied that she didn't know what the 20% people sang, which means she didn't sing in the shower. So, she is either singing in the shower or doing something else.