My 5 year old just got me with this one last night right before we fell asleep:

Him: Dad, will you remember me tomorrow?

Me: Of course!

Him: Will you remember me in a week?

Me: Yes.

Him: Will you remember me in a month?

Me: Yes..

Him: Will you remember me in a year?

Me: Yes.

Him: Knock knock.

Me: Who's there?

Him: Whaaaaaaaat? You forgot me already?