Malleable user interfaces have the potential to enable radically new forms of interactions and expressiveness through flexible, free-form, and computationally controlled shapes and displays. This work specifically focuses on particle jamming as a simple, effective method for flexible, shape-changing user interfaces where programmatic control of material stiffness enables haptic feedback, deformation, tunable affordances, and control gain. We explore the design space of malleable and organic user interfaces enabled by jamming through four motivational prototypes that highlight jamming�s potential in HCI, including applications for tabletops, tablets, and for portable shape-changing mobile devices.