The experiment to prove that materials even like insulators do have electrons and potentials. Many years ago, before electrons were understood, the charges were discovered or created by rubbing glass rod with silk or rubbing ebonite rod with fur. By rubbing the insulator like ebonite rod, negative charges or electrons are left on the surface of the insulator. The fur must have atoms teared apart leaving the positive charges in the fur and negative charges attached to ebonite rod. The rubbing has added the energy to the electrons to bring them to the surface and being taken away on a different material.
This can be understood in winter time, when you walked on the carpet and then get the electrostatic charge-shocked when your hand touched the door knob.
The charge held in the insulator cannot move or go anywhere because the material is an insulator. The insulator either got its electrons ripped off or being attached by a number of energized electrons. A single electron is small enough, light enough and have a binding electrostatic force to hold on to anything because anything will have a positive nucleus even it is electrically neutral.