  The Bethe ansatz for the one-dimensional s=1/2 Heisenberg ferromagnet is
introduced at an elementary level. The presentation follows Bethe's original
work very closely. A detailed description and a complete classification of all
two-magnon scattering states and two-magnon bound states are given for finite
and infinite chains. The paper is designed as a tutorial for beginning graduate
students. It includes 10 problems for further study.
