The Hardest Material I Ever Learned

I studied chemical engineering which was a ton of fun! haha I had to learn some pretty difficult concepts but there was one concept that haunts me till this day. It is the most difficult thing I ever had to learn in my life! What is the hardest material you had to learn?

If you want to learn more memory techniques, get your free memory program:
Also check out my podcast if you rather listen than read: