  We provide introductory explanations and illustrations of the $N$-body
hydrodynamics code Charm N-body GrAvity solver (ChaNGa). ChaNGa simulates the
gravitational motion and gas dynamics of matter in space, with the goal of
modeling galactic and/or cosmological structure and evolution. We discuss the
algorithm for leapfrog integration and smoothed particle hydrodynamics and
computer science concepts used by the program, including the binary data
structure for the particle positions. Our presentation borrows from the
doctoral dissertation of J.\ G.\ Stadel. Problems are provided in order to use
ChaNGa to learn or solidify some cosmological concepts.
