Answer:
Climbing a ladder assumes that you would touch/use each step only once. Electrons will take one layer that is empty first, and previous layers or "steps" will continue being filled with additional electrons only when every other layer is occupied by, at least, one electron.
Explanation:
A good analogy is what passengers in a bus do. Everyone tries to sit alone first and only when every pair of seats has a person already seated, people will use them as well, sitting next to each other.
