Is the Baire space $\sigma$-compact?
The Baire space is the set $\mathbb{N}^\mathbb{N}$ of all sequences of natural numbers under the product topology taking $\mathbb{N}$ to be discrete. It is a complete metric space, for example with the metric $d ( x , y ) = \frac{1}{n+1}$ where $n$ is least such that $x(n) \neq y(n)$.
A topological space $X$ is called $\sigma$-compact if it is the countable union of compact subsets.
$\omega^\omega$ is not σ-compact.
First note that every compact subset of $\omega^\omega$ has empty interior; that is, they are nowhere dense. (If $K \subseteq \omega^\omega$ is compact with nonempty interior, then there is a finite sequence $s = (\ell_0, \ldots , \ell_n )$ in $\omega$ such that $[s] = \{ x \in \omega^\omega : x\text{ extends }s \}$ is a subset of $K$. As $[s]$ is clopen it must be compact itself. However we can write $[s]$ as $\prod_{i \in \omega} A_i$ where $A_i = \{ \ell_i \}$ for $i \leq n$, and $A_i = \omega$ for $i > n$, and since $\omega$ is not compact it follows that $[s]$ cannot be compact, a contradiction!)
As a complete metric space (hence a Baire space), the Baire Category Theorem then implies that the Baire space cannot be σ-compact.