What programming concepts are the hardest to explain to non-programmers?

Classes ,Recursions,Trees,Time Complexity concepts are hard to explain

Hi, What programming concepts are the hardest to EXPLAIN to non-programmers?When I come across this kind of question, I have tried to answer what is hard. But main question is how will be explained! By my thought something is hard to explain if you do not know it well. You will just have to adjust you examples for target non programmer person so he/she will perfectly understand you.Kind regards,Nikola

Recursion functions  

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Recursion functions  and Polymorphism

Recursion, pointers and object oriented programming.

For Layman,it can be very hard to explain the object oriented principals of programming languages and the way it has been implemented in different programming languages.

well accordindg to me all programming concepts is quite hard to make a non programmers understand as one has to start with basic to make them understand. 

recursion concepts how its working

in this collection topic is very hard to explain

POO, Object Oriented Programming

it depends on how we explain. We have to explain it with real time examples so that they can understand it very well