What are some American jokes that you hear allot on tv or film that you dont understand ?

I have two that i hear a lot on tv shows or movie's where people laugh or find it funny wich i dont understand or know the reference of, can somebody explain it to a non American? 1: i have fallen and i cant get up! 2: it was a wendy's !!! 3: i want my baby back, baby back, baby back...