Physicists have created a fluid with "negative mass," which accelerates towards you when pushed. From a report on BBC: In the everyday world, when an object is pushed, it accelerates in the same direction as the force applied to it; this relationship is described by Isaac Newton's Second Law of Motion. But in theory, matter can have negative mass in the same sense that an electric charge can be positive or negative. Prof Peter Engels, from Washington State University (WSU), and colleagues cooled rubidium atoms to just above the temperature of absolute zero (close to -273C), creating what's known as a Bose-Einstein condensate. In this state, particles move extremely slowly, and following behaviour predicted by quantum mechanics, acting like waves. They also synchronise and move together in what's known as a superfluid, which flows without losing energy.