Knock knock. Who's there? Tank.

Tank who? Tank you for opening the door!

Knock knock. Who's there? Fig.

Fig who? Fig-ure it out yourself!

Knock knock. Who's there? Dew.

Dew who? Dew-n't worry, it's just a drop in.

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!