Analysis of thriller opening sequences

In media today we watched the opening sequence of three different thrillers and these were, children of men, memento and the history of violence. they all had enigmas at the beginning of them so they all involved a mystery and make the audience want to know what's going to happen next. Children of Men and Memento were my favourite beginnings out of the clips we watched. 

Children of Men is about the future and what its going to be like, so heres a brief description, In 2021, in a chaotic world in which humans can no longer procreate, a former activist agrees to help transport a miraculously pregnant women to a sanctuary at sea, where her child's birth may help scientists save the future of humankind. http://www.youtube.com/watch?v=wCTgUq6hzUk&feature=related