Let $\Omega $ countable. Why the Borel $\sigma$-algebra is the power set?

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Let $\Omega $ a countable set. Why the Borel $\sigma $-algebra is the power set of $\Omega $? Just take $\Omega =\{1,...,6\}$ Does it has even sense to talk about Borel set in $\Omega $ ? What could be an open ?