"Knock Knock - Lettuce" joke

Knock Knock!
Who's there?
Lettuce.
Lettuce who?
Lettuce try again tomorrow. Knock Knock
Who's there!
Lettuce!
Lettuce who?
Lettuce in, won't you? Knock Knock
Who's there!
Lettuce!
Lettuce who?
Lettuce in and I'll tell you!