what the?

Mother, pointing to a woman playing the musical saw: She is singing!
Son: No, She's playing the saw.
Mother: There is an orchestra playing!
Son: It is a tape.
Mother: And she is singing?
Son. No. She is sawing.
Mother: What did you say?
Son: Go, have a look.
Mother: .......
Son: And?
Mother: It's like singing.
Son: That's it, the saw.
Mother: What a nice voice she has!
Son: She is not singing. It's the saw that's singing.
Mother: No way... She is singing into the saw?
Son: No, no singing. Just sawing.
Mother: But she is opening her mouth.
Son: She is breathing.
Mother: Are you sure she is not singing?

Overheard in Union Square subway station