 Throughout the series, the theme song was a tune played on the  harmonica. It wasnt until the last season words were put to it. The  singer is John Popper of the band Blues Traveler, who previously  appeared in the episode "Of Mice and Dan". In the series finale the  song was sung slower and by another person. The lyrics are: If what  doesnt kill us Is making us stronger. Were gonna last longer Than  the greatest wall in China Or that rabbit with a drum. If its one  thing that Ive learned While waiting for my turn, Its that in each  life some rain falls, But you also get some sun. And well make out  better than okay. Hear what I say? Yeah, any day. (laugh)