For those (non-natives, etc.) who maybe aren't familiar, knock-knock jokes are one of the lowest, most basic forms of American "comedy". They follow this format:

• A: Knock, knock

• B: Who's there?
• A: [Something]
• B: [Something] who?
• A: [Dumb punchline]

Here's an example:

• A: Knock, knock

• B: Who's there?
• A: "Boo"
• B: "Boo" who?
• A: It's just me, you don't have to cry.　→　さぁ〜〜〜むッ！
  
  注：「"Boo" who」とは「Boo hoo」の同音で、「Boo hoo」とは英語の泣き[喚]{わめ}きの擬声語。

You can make up just about anything and fit it into the format of a knock-knock joke.

So does Japanese have some similar ubiquitous format for making easy "jokes"?