topology question about cartesian product I have a question about a proof I'm reading. It says: 
Suppose $A$ is an open set in the topology of the Cartesian product $X\times Y$, then you can write $A$ as the $\bigcup (U_\alpha\times V_\alpha)$ for $U_\alpha$ open in $X$ and for $V_\alpha$ open in $Y$. Why is this?
(I get that the basis of the product topology of $X \times Y$ is the collection of all sets of the form $U \times V$, where $U$ is an open subset of $X$ and $V$ is an open subset of $Y$, I just don't know why we can take this random union and say that it equals $A$.) 

You need to be careful and choose those $U_{\alpha}$ and $V_{\alpha}$ for which $U_{\alpha} \times V_{\alpha} \subset A$.
