Electrical resistance is the opposition a material presents to the flow of electric current. Even the best conductors, such as copper and aluminum, have some resistance. This is because electricity flows as a stream of free-moving electrons. As these electrons travel through a conductor, they collide with the atoms and other electrons within the material. These collisions cause the electrons to lose energy, which manifests as heat and hinders the flow of current. The more collisions that occur, the higher the resistance.