Walton Guy Pearce solves the crime with the help of Polaroids in Memento . Specifically, from moment to moment, he can identify people and places only by the stack of Polaroids he carries in his coat pocket and the details he has scribbled on them. His long-term memory is intact, however. He knows who he is, how to speak, what the ringing of a telephone means.
Memento opens with a cold-blooded murder.
An undeveloping instant photograph rolls back into a Land camera. A smoking shell casing flies back into a pistol's chamber. From these cues, we know that time and this scene are running backward, setting up the reverse narrative structure of the movie. It's a jarring, disorienting structure, and one that works splendidly for this mind-blowing film noir.