  Butz and Moerdijk famously showed that every (Grothendieck) topos with enough
points is equivalent to the category of sheaves on some topological groupoid.
We give an alternative, more algebraic construction in the special case of a
topos of presheaves on an arbitrary monoid. If the monoid is embeddable in a
group, the resulting topological groupoid is the action groupoid for a discrete
group acting on a topological space. For these monoids, we show how to compute
the points of the associated topos.
