Munkres topology book set theory chapter 1 question 10c
Let $\mathbb R$ denote the set of real numbers. for each of the following subsets of $\mathbb R\times\mathbb R$ determine whether it is equal to Cartesian product of two subsets of $\mathbb R$.
Now here $\{(x,y)| y\gt x\}$ is given as not equal to Cartesian product of two subsets of $\mathbb R$ but we can construct a product like that below:
$\{(x,y)| x \in \mathbb R, y\in B\text{ such that } B=\{y| y\gt x\}\}$
Still $x$ and $y$ are subsets of $\mathbb R$
Can anyone explain me why its not a true Cartesian product definition