The Allegory of the Cave. The dialogue between Socrates and Glaucon is probably fictitious and composed by Plato; whether or not the allegory originated with Socrates, or if Plato is using his mentor as a stand-in for his own idea, is unclear. In the dialogue, Socrates asks Glaucon to imagine a cave, in which prisoners are kept.
The d sublevel consists of five orbitals that can hold a maximum of  two electrons each. The total maximum number of electrons that the  d sublevel can hold is 10 electrons.
No matter what its shape, an orbital can only hold a maximum of two electrons at any time. At the lowest energy level, the one closest to the atomic center, there is a single 1s orbital that can hold 2 electrons. At the next energy level, there are four orbitals; a 2s, 2p1, 2p2, and a 2p3. Each of these orbitals can hold 2 electrons, so a total of 8 electrons can be found at this level of energy. In larger and larger atoms, electrons can be found at higher and higher energy levels (e.g. 3s and 3p).