Space and time are woven together to form a fourth dimension called ‘space-time’. The ‘curvature’ is a geometric property of space- time. Einstein identiﬁed this property while he was working on his General Relativity theory. The curvature of space-time is directly related to the energy and the momentum of any matter or radiation. A standard way to illustrate this idea is to place a bowling ball (representing a massive object such as the sun) onto a stretched rubber sheet (representing space-time). If a marble is placed onto the rubber sheet, it will roll towards the bowling ball, and may even be put into ‘orbit’ around the bowling ball. This occurs, not because the smaller mass is ‘attracted’ by a force emanating from the larger one, but because it is travelling along a surface which has been deformed by the presence of the larger mass. In the same way, gravitation in Einstein’s theory arises not as a force propagating through space-time, but rather as a feature of space-time itself. According to Einstein, your weight on earth is due to the fact that your body is travelling through warped space-time.