Chillin' at the Subatomic Cafe
Imagine you are a humble electron, and you walk into the Subatomic Cafe for a nice big bowl of quark soup. The room is filled with elementary particles, both bosons and your fellow fermions. Where do you sit?
Electrons are restless particles with serious intimacy issues; they need a lot of personal space. Each electron in the cafe occupies only one seat at a given table, and would shove off a stranger who tried to sit in his or her lap –- although if that person happened to be an attractive member of the opposite sex, the two could share a table.
The tables, each with a certain number of seats, represent specific energy states. Only a certain number of electrons can occupy a given state. For instance, a helium atom has two electrons, and both can occupy the same "seat" provided they are opposite "genders," determined by a mysterious property called "spin" (i.e., one is spin up, the other is spin down). In this case, two is company, and three is definitely a crowd: if a third electron tries to horn in, it will be bumped up to the next energy level.