Say you have a parallel plate capacitor where the distance between them is 1mm. If you put a metal plate directly in the middle of them with the same area and a thickness of 0.5mm you reduce the 'empty space' for the capacitors to two lots of 0.25mm distances. After doing some calculations you find that the capacitance doubles.
My question is what is the physical reason for the capacitance increasing? I was thinking that it was due to the metal plate storing additional charge hence increasing the capacitance but I am not 100% sure. Any feedback welcome!