Conservation of Electric Charge

Every object already contains huge numbers of positive protons and negative electrons, usually balanced so the net charge is zero. Rubbing two neutral objects together doesn't make charge appear from nowhere. It strips electrons off one surface and piles them onto the other. The object that loses electrons ends up positive; the one that gains them ends up negative. Add up the charge on both objects before and after, and the total is exactly the same number it started as, just arranged differently.

This is why two identical metal spheres touching behave so predictably. Metals let electrons move freely, so when two conducting spheres touch, electrons flow between them until both spheres reach the same electric potential. For identical spheres this happens when the charge splits evenly: each ends up with the average of the two starting charges. The physical electrons didn't vanish or multiply, they just redistributed until the push on them balanced out.

This idea backs up everything else in the unit. Electric field and force calculations all assume the charges you plug in are real, conserved amounts, not something you can invent to make an answer come out nicely. Later, when charge moves through circuits or between plates of a capacitor, the same bookkeeping rule applies: whatever charge leaves one place shows up somewhere else, never lost, never created, only moved.