 What makes rocket engines tough enough to withstand the incredibly high temperatures needed to escape Earth's atmosphere? A closer look reveals part of the answer. Like tiny brick walls, the boundaries between these microscopic grains help stop the motion of defects that could lead to cracking. Keeping grains small, therefore, helps keep materials like this alloy strong and intact. But under certain conditions, some grains can start to grow and fast, putting in otherwise durable material, and all it protects, at risk of serious damage. While researchers have generally attributed rapid grain growth to a single common mechanism, a team from Sandia National Laboratories suggests that not all fast-moving grains are created equally. That insight might force scientists and engineers to rethink how to make metals stronger and safer. Abnormally fast-growing grains are an important topic in materials research. Because of the risk they pose to the structural integrity of metal parts. The consensus has been that the walls of these domains move so quickly because their motion involves the glide of step-like interfaces, involving very little motion of at most a few atoms. This is in contrast to more conventional grain boundaries, which advance by the shuffling of whole groups of atoms. But this team has shown that while that may be true for many fast-moving boundaries, not all of them migrate in this way. By watching what happens at the interface of computer simulated grains, the researchers observed that fast boundaries can advance in big multi-atom shuffles, through localized rotation around fixed atoms, or through other coordinated mechanisms. There is no one mechanism that can account for the motion of all fast-grain boundaries. And more importantly, there's no way to predict what type of microstructure is likely to show rapid grain growth. These results demonstrate that much more work is needed to understand the environmental and structural factors that cause runaway grain growth in metals. Reexamining conventional wisdom could help researchers find new ways to enhance the strength and flexibility of many important materials.