Knock knock

I haven't seen/read/heard this knock-knock joke before, so I'm going to take the credit for it until someone says different. Although, coming to think of it, it's really rather obvious, so perhaps someone else has invented it. However, here it is:

Knock knock
Who's there?
Zoe
Zoe who?
Zoe Wanamaker
Wanamaker do what?

And that's how it goes :) By the way, Zoe Wanamaker is the star of the British sitcom "My Family". For the non-Brits, she's in the Harry Potter movies as Madame Hooch.