Why did the chicken cross the road? To get to the idiot's house.
Knock, knock. Who's there? Chicken.
Why did the chicken cross the road? To get to the idiot's house.
Knock, knock. Who's there? Chicken.
Listen up, buttercup. You see, the chicken crossed the road to reach the idiot's house, and then at the door the chicken showed up. The joke is that you are the idiot since the chicken is now at your door. It implies that you are the punchline, which is why it's funny... or at least trying to be.