First of all, we need that the 2 spheres need to not be in contact, otherwise their charges would balance and we would have neutral spheres. Then, rub the glass rod with silk; the glass rod is then charged positively and the silk negatively. Touch one sphere with the glass rod and the other with the silk piece. Because metal spheres are excellent conductors, they will absorb all the charge and thus they will have the same charge, but opposite signs.
