After you have learned something then that new is old and simple. Just before you learned it, it was difficult and comprehensible. When something is being explained by some one and the listener doesn’t understand, then after a long and tedious session you can hear the question “Are you stupid?” Here the fact is that the one explaining has failed and not the listener. Usually some one else enters the scene at this point and also explains the subject. No the listener says “Well, why didn’t you say so in the first place!” Here you can see that as soon as the listener got the info as he was expecting it, he understood it immediately. He who explains must understand how the listener is capable receiving information. Only then he can give his information to the receiver. If the explainer can’t understand this, then the listener can’t understand what is being said.
No one is stupid. People just don’t know how to make themselves clear.