What was the hardest concept for you to grasp when learning Object-Oriented Programming?

I would say about 99% of developers use OOP in their day-to-day programming and perhaps even a higher percentage than that learn OOP at the very least.

I'm curious to see if there is a concept or idea specific to OOP that many people agree was hard for them to learn at first.

For me, understanding the difference between class variables/methods and instance variables/methods was a bit of stumbling block to me. I used to not understand why you'd want to make a method static.

How about you? When you learned OOP or if you're currently learning it, what concepts did/do you find most challenging to understand?