Well, Anything is simple if you know, not only Polymorphism. The critical part of Polymorphism is not in coding. The critical part lies in Designing it. Rather, think like polymorphism is difficult. The idea of naming the same task with different form is difficult. I have explained enough (at least to my extent) in these videos. I know changing somebody in a thought process is little difficult than to make them do it. Humans are more trained to follow instructions. For that I have practicals to follow up with theory.
Sometimes it is not possible to listen once and understand the concept. So please go twice. Do not hurry up on these. These are fundamentals. See the practical and practice it twice. It needs your attention seriously.