Gravitational lensing is now regularly observed by astrophysicists, including the lensing of distant quasars by galaxies, and the lensing of stars in the galactic nucleus and in the Larger Magellanic Cloud by more nearby stars. The lensing is readily understood with the help of Fermat’s principle of stationary time to find the paths of light rays. Conventional discussions of Fermat’s principle in introductory physics and optics courses may easily be extended to include gravitational lensing, to show for example that even the simplest gravitational lens can produce multiple images of the same source.