Knock knock. Who's there? Axe.

Axe who? Axe-actly! I've been axe-pecting you to open up, 'cause I just had an axe-cellent idea for an axe-traordinarily sharp joke!

Knock knock. Who's there? Sewers.

Sewers who? Sewers *nice* of you to ask, but these puns are really *sewers*-ing my patience, making me think it *sewers*ly time to *sewers*-t out a new joke writer!

Knock knock. Who's there? Dough.

Dough who? Dough-n't worry, I knead to come in.

Knock knock. Who's there? A door.

A door who? A door has no 'who,' it's an inanimate object.