I've always seen it instructed as think, but because of the way we talk, most people hear thing -- so it's kind of morphed into "that's how I heard it growing up." Someone once told me about the phrase "used to" and the (I'm sure very valid, but I didn't care) reason it's supposed to be use to. I didn't care enough to change my way of using it, when the whole world seems to employ the phrase the way I do, so I think people are safe with whichever they choose.