New analysis of an extraordinarily deep Chandra image of Cassiopeia A shows that this supernova remnant accelerates electrons to enormous energies. The blue, wispy arcs reveal the acceleration is taking place in an expanding shock wave generated by the explosion that destroyed the progenitor star. This acceleration is close to the theoretical limit and provides strong evidence that supernova remnants are key sites for generating cosmic rays, mysterious high-energy particles that bombard the Earth.