As Ralph Colby peers at the microscope image in front of him, he thinks he can make them out—”shish kebabs,” as polymer scientists call them. Nobody knows for sure what they are, but these shapes that appear at seemingly unpredictable times when certain plastics cool have a big impact on the overall properties of plastics. It’s not a big deal when a plastic fork breaks, but if a bearing cage on an airplane were to break, it could put people in danger.