Here's a short story:

Set theory is a branch of mathematical logic that studies sets, which informally are collections of objects. Although any type of object can be collected into a set, set theory is applied most often to objects that are relevant to mathematics. The language of set theory can be used in the definitions of nearly all mathematical objects. 

The modern study of set theory was initiated by Georg Cantor and Richard Dedekind in the 1870s. After the discovery of paradoxes in naive set theory, such as the Russell's paradox, numerous axiom systems were proposed in the early twentieth century, of which the Zermelo–Fraenkel axioms, with or without the axiom of choice, are the best-known. 

Set theory is commonly employed as a foundational system for mathematics, particularly in the form of Zermelo–Fraenkel set theory with the axiom of choice. Beyond its foundational role, set theory is a branch of mathematics in its own right, with an active research community. Contemporary research into set theory includes a diverse collection of topics, ranging from the structure of the real number line to the study of the consistency of large cardinals. 

Mathematical topics typically emerge and evolve through interactions among many researchers. Set theory, however, was founded by a single paper in 1874 by Georg Cantor: "On a Property of the Collection of All Real Algebraic Numbers". (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: who started the study of set theory?
Guess: Georg Cantor and Richard Dedekind

Question: when?
Guess: in the 1870s

Question: what is set theory?
Guess: branch of mathematical logic

Question: what are sets?
Guess: collections of objects

Question: what paper founded set theory?
Guess: On a Property of the Collection of All Real Algebraic Numbers

Question: when was it written?
Guess: 1874

Question: by who?
Guess: Georg Cantor

Question: what is a current topic in set theory?
Guess: structure of the real number line

Question: can any object be in a set?
Guess: yes

Now answer the following question in the same format.

Question: what objects are usually part of set theory?

objects that are relevant to mathematics