I've been reading recently about "rat mills". I don't really understand what they are, but I can tell they're bad. I've also heard/read that buying animals from pet stores are bad and it's better that I buy my rats from a shelter. What exactly is true? The 2 rats I bought are healthy considering they were from a "pet store" which is Petsmart. Are all rats and such from pet stores taken from the mills or are the pet stores reliable? I'm totally confused about the mills and would appreciate an explanation.