I once met a girl who didn't know how to tell knock-knock jokes. I didn't really like her, but one day I was humoring her enough to have a small conversation with her and she decided to test one out on me. And this, simply, is how it went:
Her: Knock-Knock!
Me: Who's there?
Her: Birthday!
Me: Birthday who?
Her: Birthday cake!
I have a tendency to laugh at the most inappropriate times, but I can keep it in when it counts, fortunately...just long enough for me to finish the conversation and leave the vicinity to come within an inch of turning my lungs inside-out...