Everything around us is made of atoms.Atoms themselves are made of still smaller particles called protons, neutrons and electrons.Electrons are the tiniest of these particles.Each one of them carries the smallest possible amount of negative electricity,and even tiny bits matter contain billions of them.In most materials,and especially in good insulators like glass or plastics,electrons are held firmly in place by the heavier,positively charged protons.But some materials,mostly metals,contain electrons that have enough energy to roam about.This allows the electrons to transport electricity from one place to another,and means that metals are good conductors.They are used for this purpose in all electrical systems.And when metals are heated,this gives their electrons still more energy,sometimes enough to break free completely,like rockets escaping the pull of the Earth.