As regards "Knock knock..."
An old lady had a parrot and a TV. One day, the TV broke down, so the old lady called an engineer to repair the TV. However, shortly before the engineer came, the old lady had forgotten the appointment and went out shopping, leaving the parrot in charge.
A very few minutes later, the TV engineer rang the bell. From inside, he heard a voice calling, "Who's there?"
He replied, "The TV engineer."
Again, a voice from within, "Who's there?"
Slightly exasperated, the caller replied, "The TV engineer..." -
"Who's there?" - "THE TV ENGINEER!!" - "Who's there?" - Yelling at the top of his voice, "THE TV ENGINEER!!" - "Who's there?" (engineer has a heart attack and faints in front of the door)
A few minutes later, the old lady returns, sees the unconscious man in her front garden and exclaims, "Who's that?" - "The TV engineer, the TV engineer..."