Intersecting Subsets

Let $S_n = \{1,2,\dots, n\}$. Two subsets $A,B \subseteq S_n$ are selected independently and uniformly at random from the set of all possible subsets of $S_n$. Note that $A = B$ is possible. Define the random variable $N_n = |A \cap B|$. Compute $\dfrac{\mathbb{E}\left[ N_n \right]}{\var{N_n}}$ when $n = 25$.