you know the songs, i'll we there for you, moving on up, come on and get happy. they were the songs to classic shows we love. that begs the question, where have all the theme songs gone? we sent don lemon to find out. one thing he found out is writing songs is hard but writing tv theme songs is even harder. take a look. >> hello, everyone. i spoke to tv theme song composers and legendary actors to try to get to the bottom of this question. what happened to tv theme songs? what happened? we all used to sing them. but first i had to ask them what was their favorite tv theme song of all time. mine is "movin on up," the jeffersons from the '70s. listen to theirs. >> outside of your own amazing show, which i love, i have to say that, what is your favorite tv theme song?