  This note is a pedagogical mini review on the muon anomalous magnetic moment
($g-2$), translated and adapted from our article published in Modern Physics 4
(2021) 40-47. The contents include: (i) The magnetic moment of an
electric-current coil; (ii) The magnetic moment of a charged lepton estimated
as a classical charged ball with spin; (iii) The magnetic moment of a charged
lepton from Dirac equation with electromagnetic interaction; (iv) The $g-2$ of
a charged lepton from QED beyond tree level with effective couplings; (v) The
measurement of muon $g-2$; (vi) The muon $g-2$ in low energy supersymmetric
models. Finally, we give an outlook.
