Our device is based on a physical phenomenon called electrowetting: If you apply electrical voltage to certain liquids, the liquid moves. This means you have converted electrical energy (the current) to mechanical energy (the liquid in motion). We reversed the process, forcing liquid to move over electrodes. In the shoe, you have two flexible plastic bladders, one under the heel and the other under the toe. The bladders are filled with a mixture of oil and water and connected by a thin, snaking tube. When you step down on your heel, you compress the rear bladder, and several milliliters of liquid travel through the tube to the front bladder. Step on the toe, and the process is reversed.