Axiom:Axiom of Pairing

For any two sets, there exists a set to which only those two sets belong.

$$\forall A: \forall B: \exists x: \forall y: \left({y \in x \iff y = A \lor y = B}\right)$$

It is otherwise known as the Axiom of the Unordered Pair.

Thus it is possible to create a set containing any two sets that you have already created.