This project presents a weight-changing device based on the transfer of mass. We chose liquid metal and a bi-directional pump to inject into or remove from from the mass of a target object. The liquid metal is about six times heavier than water, and is thus suitable for effective mass transfer. We also combine the device with a dynamic volume changing function to achieve programmable mass/volume at the same time. We explore three potential applications enabled by weight-changing devices. This technique opens up a new direction in human science regarding user interfaces with dynamic weight/volume changing.