Why is it called singing when you go to the toilet in the car?

Not only in the car, in fact, going to the toilet to urinate can be called singing.

This is all I know,

The name comes from a joke,

It is said that the mother took her son as a guest, fearing that the child would say something uncivilized in front of his master, she said to her son, When you get there, if you want to pee, just tell me that you want to sing.

The child remembered, and as a result, the child didn't want to pee all day.

His mother forgot about it, too

But when sleeping at night, my son wants to pee.

So he said to his mother: Mom! Mom! I want to sing!

Mother scolded: this child! What song to sing so late! Everyone is asleep! It will wake people up! No singing!

My son stopped talking, but he was in a hurry! I couldn't hold it any longer. After a while, I quarreled with his mother: Mom! Mom! I still want to sing!

Mom is depressed but has to say: good good! If you really want to sing, sing softly in your mother's ear!

...................

That's it, so ... that's what you call singing.