Why do you call going to the bathroom singing?

That's what tour groups call it. A habitual name for a tour guide is probably because it is more elegant and less explicit. It is generally understood that going to the toilet may be associated with whistling or humming. Later, everyone joked about it and thought it was more elegant and humorous.

Another way of saying it is a short story, as follows:

Originated from a joke, children often say that I have to pee when eating at home, and once grandma took her grandson to a high-end party. In order not to be rude, she taught the children to say, "If you have to pee when eating out, just say grandma I want to sing." Grandson remembered and successfully attended the party. Everyone fell asleep at night, and the grandson said eagerly, "Grandma, I want to sing." Grandma, I want to sing. "What do you want to sing in the middle of the night?" Grandma said, "I can't hold back, but I want to sing." Grandma can't say, "Then sing in grandma's ear, but in a low voice." . . . . .

One person is called solo, two people are called duet, more than three people are called chorus, and the roadside in the mountain is called folk song.