Full Idea
The muon, like all fermions, spins - and because a spinning electric charge generates a magnetic field all fermions act like tiny bar magnets.
Gist of Idea
Spinning electric charge produces magnetism, so all fermions are magnets
Source
Gavin Hesketh (The Particle Zoo [2016], 11)
Book Reference
Hesketh,Gavin: 'The Particle Zoo' [Quercus 2016], p.272