Knock knock. Who's there? Cows go.

Cows go who? No, cows go 'moo'.

Knock knock. Who's there? Sue

Sue who? Sue-per excited to meet you, it's a shoe-in for a new friend!

Knock knock. Who's there? A series of percussive sounds.

A series of percussive sounds who? A series of percussive sounds doesn't usually introduce itself, but I felt it was important to be precise about what just occurred.

Knock knock. Who's there? Wendell.

Wendell who? Wendell you give in to the sheer pun-ishment of this joke and finally admit it's been weighing on you, a real heavy hitter in the groan department, making you wonder when 'til the end of this tale of wordplay woe?