Monomorphisms and epimorphisms in the category of Boolean algebras A Boolean algebra is a ring with unity all of whose elements are idempotent.
We regard a zero ring $0$ as a Boolean algebra.
Let $\mathcal{B}$ be the category of Boolean algebras.
A morphism in $\mathcal{B}$ is a homomorphism of rings preserving unity.
Is a monomorphism in $\mathcal{B}$ always injective?
Is an epimorphism in $\mathcal{B}$ always surjective?
 A: In any algebraic category monomorphisms are injective, because the forgetful functor to the category of sets has a left adjoint (the free algebra on a given set) and therefore preserves monomorphisms. In the case of boolean rings, the free boolean ring on one generator is $\mathbb{F}_2[x]/(x^2-x) \cong \mathbb{F}_2 \times \mathbb{F}_2$, and hence the free boolean ring on a set $B$ is $\bigotimes_{b \in B} \mathbb{F}_2[x]/(x^2-x)$, where this is the (possibly infinite) tensor product as $\mathbb{F}_2$-algebras, which is here also the coproduct in the category of boolean rings.
I don't know right now if epimorphisms are surjective, but using Stone duality this is equivalent to a pure topological question: If $A$ is a closed subspace of a totally disconnected compact Hausdorff space $X$, does every open-closed subset of $A$ lift to a open-closed subset of $X$? If I remember correctly, there are (exotic) counterexamples.
A: In the category of Boolean algebras, epimorphisms are surjective.  See B. Banaschewski, "On the strong amalgamation of Boolean algebras," Algebra universalis 63, pp. 235-238 (2010)
