Prince Harry revealed that his father was so set in his habit, that young royal followed the example, and didn't notice how it became his second nature.
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However, whenever Harry would do it in front of his classmates, they immediately started to mock him, seeing that he's a royal and it was apparently low for someone of his status.
Duke of Sussex remembered those cruel days:
I used to get taken the mickey out of at school for just picking up rubbish. When you go for walks anywhere, if you see something, it stands out, you pick it up. Before you know it, somebody [says], 'What are you doing?' I've literally done this because I am programmed to do it because my father did it. And actually, we should all be doing it.