whats 9 plus 10

9 + 10 = 19. He escuchado a algunas personas decir que es 21. Yo no lo entiendo. ¿Puede alguien explicar?

9 + 10 = 19. I have heard some people say that it is 21. I don't get it. Can anyone explain?

9+10=19. or the joke, then the answer is 21

google it.