Television Confuses Me

I don't understand how reruns work. For the last two months or so, I have been confused by the tv schedule. It seems like every other week is a new episode and every other week is a rerun. I get all excited to watch certain shows, and then they either aren't on (like New Christine last week) or they are reruns (like Grey's Anatomy last week and the Monday CBS comedies today). I wish shows would just do new shows for a couple months straight and then do a bunch of reruns. At least then I would know what to expect.