Knock, knock. 

Who’s there?

Lettuce.

Lettuce who?

Lettuce in and you’ll find out!

Explanation: Knock-knock jokes play with words: words that sound the same but are different or words that have more than one meaning.  In this joke the jokester is playing with the words “let us” and “lettuce.”  It is funny because the listener assumes the word is lettuce.