For HP fans only. Last night my sister and me were watching " HP and the POA " on TV. In one of the scenes Harry and Hermione use an instrument known as the time turner to travel back to the past and try to save Buckbeak and Sirius Black . As they use the time turner, they find that several of the unusual things that had happened in the "past" , like the stone being thrown into the window to alert them etc, were all part of their doing using the time turner, and so they repeat what they had seen or heard in the "past" while they are using the device. In effect, they don't change anything that had already happened in the past. This becomes a paradox, if I may use the term, since had they used the time turner any no of times they would also alter the past, which is not possible (suppose using the time turner they throw two stones instead of one through the window, but we already know that there was only one stone in the past). Also a similar argument would mean that Sirius and Buckbeak were already saved( just like the stone was thrown ). So what use is the time turner anyway? Or is there something that I am missing ?