For me, that's math.

I'm not really a mathematical prodigy, it often takes me hours to completely understand a chapter in a math text(which is often just 2-5 pages). Yet, the more I read it...the more I want to learn of it.

Like, it took me a while to digest the proof of why there are infinitely many prime numbers. It also took me a while to fully understand and illustrate what an abelian group is. Pre-Abstract Algebra and Number Theory are by far my hardest math classes...and I haven't even taken Euclidean Geometry, Combinatorics, Applied Probability Theory, or Calc4 yet. I'm kind of scared that I'll somehow fail these classes and watch my degree go down the drain...yet I'm kind of determined to see it through...all so I can teach low-level math like AP Calculus and high school Algebra...ironic isn't it?