  To constitute atoms of a $\sigma$ algebra is not a easy task due to the large
number of its elements. However, determining them via generators seems a
feasible and simple way since most $\sigma$ algebras are generated by their
smaller proper subsets. Precisely, under some conditions each atom of a
$\sigma$ algebra equals the intersection of the elements containing a point of
the atom in the generator. In this paper, a very weak sufficient condition for
determining atoms by the generator is presented. The condition, though not
being a necessary one, is shown to be almost the weakest one in the sense that
it can hardly be improved.
