What is common among all these photographs of Olympic athletes?
Yup. They have all won something.
Duh! Try something else.
Aren't you thinking why they are all biting their medals?
They do look jubilant and seem to say 'I won it and it is all mine'. Is that the reason?
Or is it just a rigmarole that all the photographers want to follow, just because it suits them? Makes them alive and shining?
Well you are wrong.
Apparently it is an age old technique to check if the metal was real.
Even today, the medals are often plated with 24 carats gold, and are not fully made of gold, since 1912. Gold being a soft malleable metal leaves a mark when you bite it, if it was pure. So the test still works.
Never stop learning!