Physicists use large accelerators to make high-energy particles collide.
By studying these collisions, physicists out about the particles that make up our world. In LEP, beams of electrons and positrons travel in opposite directions round a circular tube, emptied of air. The particles travel round the collider 11,000 times a second - at almost the speed of light. When the electrons and positrons have been accelerated to high energies, focussing magnets make the beams collide. The collisions are too small to investigate directly.