This Demonstration shows the reflection and transmission coefficients for a Dirac particle with spin-up incident on a square barrier of variable height The energy of the particle is fixed at 1 unit ), but its mass is allowed to vary from 0 to . What we see is that the negative energy solutions of the Dirac equation become accessible on the barrier side, thus allowing both reflection and transmission coefficients to increase beyond 1: Klein's paradox! The phenomenon can be explained by particle-antiparticle pair creation at the potential barrier.