I have read that INTP's enjoy studying and/or analyzing "systems". I'm not sure how much this applies to other NT's, but if it applies to you, then feel free to answer.
Some of the so-called "systems" I have spent a lot of time studying and analyzing in my life are:
-Genetics (heredity, how dominant and recessive genes manifest themselves, etc.)
-The U.S. political system (rotation of election cycle within Congress, number of representatives for each state, etc.)
-Geography (state capitals, national capitals, the interstate highway system, etc.)
-military structure (chain of command, how the different branches compliment each other, etc.)
-state university systems (as a whole and also their individual degree programs - when I was an undergrad I think I enjoyed analyzing the various programs that were available to me more than I enjoyed commiting to one)
-the major sports leagues (knowing how all the playoff systems work and knowing all the teams/divisions, how often they play each other, etc.)
-gambling games (horse racing, roulette, poker, etc.)
-IQ scores and distributions
-corporate/organizational structures (how departments interact with one another/compliment each other as well as corporate hierarchies and individual industries like the soft drink industry, for example)
Sometimes I find myself looking for some new system to study, but finding nothing of great interest. What are some of the more interesting/fascinating systems that you have studied or learned about that have held your attention for relatively long periods of time (or on-and-off throughout your life)?