Charging by Induction
Rub a cotton cloth on a glass rod. This would give the rod a positive charge. Then bring the rod close to a metal ball, but not touching it.
Metal is a conductor. This means that some electrons are free to move around in the ball. When the rod comes close, the electrons are attracted and move towards the rod.
So negative charge accumulates on the side of the ball closer to the rod. The other side of the ball would then be short of electrons, and so become positively charged.
This method of producing opposite charges on two sides of a metal is called induction. When the rod is removed, the electrons move back and the charges become zero again. There is a way to use induction to produce a non-zero charge on a metal even after the rod is removed.
Instead of one metal ball, use two metal balls. Put the two balls together, touching. Bring the positively charged rod close to one sphere, but not touching it.
Electrons in the metal are attracted to the ball closer to the rod. The other ball is left with a shortage of electrons.
Now move the two balls apart so they no longer touch. (The balls must be insulated from your hand, or charges will leak into your hand.) Then remove the rod.
Electrons from the ball closer to the rod tries to move back to the other ball. It cannot, because the other ball is no longer in contact.
So it becomes negatively charged, and the other ball positively charged. This is called charging by induction.