Snapping my fongers. People tried to teach me throughout my childhood, but zi just couldn't get it. Then, when I was maybe sixteen or so, I had a dream that someone taught me to snap, and in the dream I could do it! I woke up from that dream snapping my fingers, and have been able to do it ever since.
I guess my unconscious mind finally put all the pieces together and it all finally just, pun intended, clicked.
I remember when I was in primary school it was told that it wasn't actually the amplified sound of your finger slapping against your palm and cavity formed by other fingers, but pockets of nitrogen in your knuckles. This made it harder for some kids to do it.