Ketchup has a single duty but it misbehaves. It is notoriously difficult, and often messy, to get out of the bottle. But Cambridge Professor Michael Cates understands it. Soft matter—from shampoo to chocolate—is composed of tiny particles, some of which may act like solids when the rest is running like liquids. In disagreement, they create jams, like the driver who suddenly hits on the brakes and stops everyone else.