We present a novel string mechanism for the design of pop-up cards. We apply this mechanism to create three new pop-up card designs: (1) an asymmetrically folding cube, (2) a rhombicuboctahedron, and (3) a sliceform character, all of which use string to induce the pop-up. Easily adaptable, our technique can be used in future pop-up card design. Moreover, the questions we pose in the process may have even broader implications in robotics and engineering.