When you complete your PhD thesis, you have to complete what is called the ‘defence’ of your thesis. This is also sometimes called the ‘viva’. This is where specialists in your field of study who have read your thesis ask you a series of questions to ensure that your argument is both your own and suitably solid. A PhD thesis must be a significant contribution to its field of study, and these people are there to ensure that this is true.
However, many people have been taken by surprise the snake fight part of your thesis defence. As explained clearly in this informative page, the stronger your thesis the smaller the snake that you must defeat in combat. All people with PhDs have done this but many do not speak of it, usually due to feeling that this is an old-fashioned tradition that they would rather not speak about. Check out the link for more information. Personally I can only suggest that you wear very think gloves and a sturdy pair of boots.
[Thanks to Val for the link.]