Why are Olympic gold medalists always biting their medals?
Simple answer: Photographers tell them to. No, really.
Longer answer: Historical throwback to testing whether gold coins were actually solid gold, instead of being gold-plated lead (or some other less valuable metal). Olympic gold medals are not solid gold any longer, but some pro photogs think it makes for a fun photo.
Watch as Today I Found Out deep-dives into the reasoning behind this photographic phenomenon: