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.
1 Helium has 2 electrons; the 2 electrons both occupy the s sublevel in principal energy level 1.  Lithium has 3 electrons; 2 of the 3 electrons occupy the s sublevel in principal energy level 1. The 3rd electron must go in the next available sublevel, 2s.