An in-joke, also known as an inside joke or a private joke, is a joke whose humour is understandable only to members of an ingroup, that is, people who are in a particular social group, occupation, or other community of shared interest. It is an esoteric joke, i.e., it is humorous only to those who are aware of the circumstances behind it. In-jokes may exist within a small social clique, such as a group of friends, or extend to an entire profession. An example is: Q: What's yellow and equivalent to the axiom of choice?A: Zorn's lemon. Ethnic or religious groups may also have their own in-jokes.