The soft-sphere collision gate proposed by Margolus gives us a rather realistic representation of interaction gates with real-life physical and biological bodies (figure ). Logical value x = 1 is given by a ball presented in input trajectory marked x, and x = 0 by the absence of the ball in the input trajectory x; the same applies to y = 1 and y = 0, respectively. When the two balls, approaching the collision gate along paths x and y collide, they compress but then spring back and reflect. As a result, the balls come out along the paths marked xy. If only one ball approaches the gate, that is for inputs x = 1 and y = 0 or x = 0 and y = 1, the balls exit the gate via path xy (for input x = 1 and y = 0) or xy (for input x = 0 and y = 1). Soft-sphere-like gates have been implemented using microlitre sized water droplets on a superhydrophobic copper surface . Using channels cut into the surface, not-fanout, and-or and flip-flop gates were demonstrated. The water droplets only rebounded in a very narrow collision property window, and a thorough & complete superhydrophobic surface treatment was required.