These photos are shadowgraphs of a hydrogen flame exploding inside a balloon. The shadowgraph optical technique highlights density and temperature variations through their effect on a fluid’s refractive index. Here we see that the hydrogen flame has a strong cellular structure and is more turbulent than a methane flame. The cellular structure is a sign of an instability in the curved flame front. The instability and accompanying cellular appearance are a result of the complicated transport and reaction of fuel and oxidizer inside the flame. (Photo credits: P. Julien et al.)