Energy is carried from the sun to the earth or from an open fire to hand by electromagnetic waves. More surprisingly is the fact that electromagnetic waves also transport linear momentum. This means that it is possible for electromagnetic waves to exert a force, and hence a radiation pressure, on an object, e.g. by shining light on it. Such forces must be small as we do not, after all, get pushed backwards when we raise a window blind and let in the sunlight. Radiation pressure is partly responsible for the way the tails of comets stream away from the Sun.