  By a ring groupoid we mean an animated ring whose i-th homotopy groups are
zero for all i>1.
  In this expository note we give an elementary treatment of the (2,1)-category
of ring groupoids (i.e., without referring to general animated rings and
without using n-categories for n>2).
  The note is motivated by the fact that ring stacks play a central role in the
Bhatt-Lurie approach to prismatic cohomology.
