Since it began operation in 2009, SLAC’s Linac Coherent Light Source (LCLS) has allowed scientists to make new types of X-ray measurements that were once thought unattainable by delivering one trillion X-ray photons ­– particles of X-ray light – in incredibly short bursts of less than 30 femtoseconds, or millionths of a billionth of a second. We promised that this astonishing quantity of photons, delivered in such a small slice of time, could capture the motions of atoms in chemical reactions. Now we have used this capability to make a “molecular movie” of a molecule undergoing a chemical reaction from start to finish, with frames just a few femtoseconds long. We assembled the movie by taking individual X-ray snapshots of the molecules that show the positions of their atoms at each moment in time. Comparing these results to computer simulations of the reaction, we determined the routes the individual atoms followed as the molecule rearranged. This is the first step in developing robust methods for visualizing molecular motions in chemistry, biology, and materials science at the atomic scale. Please enjoy the movie.