Some mindless internet surfing brought me near this one:

Will you remember me in an hour?
Yes.
Will you remember me in a day?
Yes.
Will you remember me in a week?
Yes.
Will you remember me in a month?
Yes.
Will you remember me in a year?
Yes.
I think you won’t.
Yes, I will.
Knock knock
Who’s there?
See? You’ve forgotten me already!

Want more? Then look here.
Don’t want more? Then stay at the BaseBlog site – I’ll try not to tell you any more knock-knock jokes. 😉