The Aptitude Test

 Rules
1. You cannot be tested by someone of your faction
2. You cannot prepare for the test in any way
3. Only one student per room


It is taken by sixteen year olds
It is taken in rooms used specifically (and only) for the Aptitude test. The rooms are covered in mirrors, which is a change for Beatrice, and Abnegation who only sees her reflection once every three months.
In the room, there is a reclined chair (like at the dentists) with a machine next to it.
The students are given a vial of clear liquid to drink on blind faith--the test giver is not allowed to answer questions about the contents of the vial or the procedure to take place.

From what I can tell, the vial must have contained some sort of hallucinogen, or something that, when combined with the effects of the machine (which I haven't any clue about what part the machine plays-- maybe recording her responses?) causes scenarios to be played in Beatrice's mind.

She gets tested in a few different ways, demonstrating stubbornness and questioning when she doesn't pick either the knife or the cheese from the basket, kindness and submissiveness when she gets down in front of the dog, bravery when she tackles the dog to save the little girl, and bravery and levelheadedness when she doesn't tell the man on the bus about possibly knowing the murderer.

Apparently, after Beatrice took the vial, Tori (the tester) attached wires to Beatrice's head.
Once Beatrice regains consciousness or whatever, Tori tells her that her results are "perplexing".
Tori soon explains that every choice Beatrice made in the scenarios was supposed to have ruled out all factions but one as possibilities, or given conclusive evidence of one specific faction. Beatrice's test only ruled out two of the factions: Amity and Candor.
This means that Beatrice has equal aptitude for Abnegation, Dauntless, and Erudite.

"People who get these kinds of results are..." She looks over her shoulder like she expects someone to appear behind her. "...are called... Divergent." She says the last word so quietly that I almost didn't hear it, and her tense worried look returns. 

"This is different. I don't mean you shouldn't share them now; I mean you should never share them with anyone, ever, no matter what happens. Divergence is extremely dangerous."