The combination of attractive and repulsive forces in a concentrated beam of laser light is sufficient to grab and hold tiny particles. Known as "optical tweezers" or "optical traps", scientists are now harnessing these extraordinary effects to investigate the behaviour of matter at a nano-scale. Peter Evans talks to scientists who are developing and using these optical tweezers in a variety of different fields - from stretching a single fibre of muscle to building tiny motors that one day might power a "lab on a chip". Producer John Watkins