“The answer to this riddle has a hole in the middle,  
And some have been known to fall in it.  
In tennis it's nothing, but it can be received,  
And sometimes a person may win it.  
Though not seen or heard it may be perceived,  
Like princes or bees, it's in clover.  
The answer to this riddle has a hole in the middle,  
And without it, one cannot start over.” 

This is a riddle that comes from Trenton Lee's book, The Mysterious Benedict Society and the Prisoner's Dilemma