"knock knock... banana.... banana who" joke

[rou]

A typical version of this joke goes like this:
Knock, knock
Who's there?
Banana
Banana who?
Knock knock
Who's there?
Banana Who?
Knock knock
Who's there?
Orange
Orange who?
Orange you glad I didn't say banana?
Now, my question is this: What's so funny about it? So bored that anything other than what we are having starts to sound funny? And, "Orange who" is there to mislead some people to hear it as "Aren't you" (I did, but then my oral English sucks)?

[Casiopea]

It's a play on sound. "o" and "a" of orange and "a" and "e" of aren't are reduced schwa, an unaccented vowel, represented below by the letters uh:

uh-ruhn-chew: Or-an-ge you glad ...
uh-ruhn-chew: Ar-en-'tyou glad ...

The schwa is the vowel sound in many lightly pronounced unaccented syllables in words of more than one syllable. It is sometimes signified by the pronunciation "uh" or symbolized by an upside-down rotated e. See symbol here.

[rou]

It's a play on sound. "o" and "a" of orange and "a" and "e" of aren't are reduced schwa, an unaccented vowel, represented below by the letters uh:
uh-ruhn-chew: Or-an-ge you glad ...
uh-ruhn-chew: Ar-en-'tyou glad ...
The schwa is the vowel sound in many lightly pronounced unaccented syllables in words of more than one syllable. It is sometimes signified by the pronunciation "uh" or symbolized by an upside-down rotated e. See symbol here.

So you are saying that the funny bit of it is that the person who kept on asking "Who's there?" will eventually realize that the door knocker didn't say "Aren't you glad I didn't say banana?" and get a laugh outta that?
Anyway, thank you for your prompt reply - I thought I had to wait a bit longer. :)

[Casiopea]

You're welcome.

psst. Humor is subjective.

[rewboss]

The "knock-knock" joke is a staple of childhood homour. It always (well, nearly always) has this form:

A: Knock knock.
B: Who's there?
A: [name]
B: [name] who?
A: [play on words based on the name]

For example:

A: Knock knock.
B: Who's there?
A: Ginger.
B: Ginger who?
A: Ginger [= did you injure] yourself when you fell off the wall?

In my stand-up comedy days, I once embarrassed the emcee by telling a knock-knock joke. It went like this:

Me: Do you like knock-knock jokes?
Emcee: Not really, no.
Me: Never mind. I have a really good one for you. You start.
Emcee: Oh, OK. Knock knock.
Me: Who's there?
Emcee: .....er.......

I don't think she ever forgave me for that.