Every once in a while, I remember, from years past, some weird card skill I either had or was aware of, that I at some point forgot about. The other day I was thinking about something I used to practice: clocking a deck of cards. For those who aren't aware, clocking a deck is essentially the ability to have a card removed, and then by spreading through the cards and doing some simple grade-school level math, be able to identify which single card is missing. I used to practice this about 6 or 7 years ago, and I can't remember the first thing about how to do it. To my surprise, searching for this on the internet doesn't bring up much information. My question is this: I have no idea where I learned this, does anyone know where I can find the method? I assume it's like forgetting some calculus equation. If it was written out in front of me I would be right back on track.