If a used comb is brought close to pieces of paper, they are attracted to it. If a balloon is rubbed against a wall, it sticks to it. These are examples of the phenomenon of static electricity, which has been known to humankind for thousands of years. For example, ancient Greek literature records static electricity experiments on fur and amber. When a piece of amber is rubbed vigorously with fur, an attractive force develops between the two. If they are then separated, each attracts other objects, like paper. However, two such pieces of amber repel each other, as do two such pieces of fur. However, metallic objects do not experience electrical forces. These observations suggest that the electric property of matter, called the electric charge, comes in two types: positive and negative. If the interacting objects carry the same sign of electric charge, they repel each other. If they carry the opposite sign, they mutually attract. Electrical forces can act without physical contact between charged objects.