Anita Poo

Okay, so listen up, because this is important. The joke uses a "knock knock" format, right? The last line is supposed to be a punchline that uses the name "Anita" to sound like another phrase. In this case, "Anita poo" sounds like "I need a poo." People think needing to go to the toilet is funny because you are immature and haven't grown up yet. You should know this because you are probably the same age as the person who wrote the joke.