Grounding

The earth is enormous compared to any lab object, so moving a handful of electrons onto or off it barely dents its charge density. That means its potential stays essentially fixed, and we call that fixed value zero. When you run a wire from a charged conductor to the ground, you are forcing the conductor to share that same potential. Charge flows through the wire until the conductor's potential matches zero, then the flow stops. On an isolated positive sphere with nothing else around, this means electrons flow up from the ground into the sphere until its charge is fully neutralized — charge and potential both land on zero together.

That tidy picture breaks when another charged object is nearby. A charged rod held near the sphere induces its own charge redistribution, and the sphere's potential now depends on both its own charge and the rod's influence. Grounding still drives the potential to zero, but to hit that target the sphere may need to keep some leftover net charge, opposite in sign to the nearby rod. Cut the wire at that instant and the charge gets trapped — nonzero, even though the sphere was just grounded.

This is exactly what happens inside a capacitor when one plate is grounded: grounding fixes that plate's potential at zero, and charge rearranges on its surfaces so the whole system satisfies that constraint along with whatever charge sits on the other plate. Understanding grounding this way sets up capacitor charge distributions, where induced charges on grounded conductors are never just zero by default.