Maths is basically magic. So it's no surprise that a clever use of the Fibonacci numbers — a series of numbers (1, 2, 3, 5, 8, etc.) where each number is the sum of the two preceding numbers — and a super-slick shuffling method can combine for a card trick that makes it impossibly easy to guess the number and suit of the two cards you're holding.
Watch as "Mathemagician" Colm Mulcahy explain his Little Fib card trick below. The idea is simple: arrange a deck of cards so that five or six Fibonacci numbers (with suits that you memorized) are on top. Ask them to pick two cards and to tell you the sum of their cards (Aces are one, Kings are 13). Shuffle the cards back in randomly. Select their cards from the deck since you know the two cards that were added together because they were the Fibonacci numbers and each sum is unique to only two specific numbers (or in this case, cards). Genius!
Unrelated: I love watching goofy mathematicians use their prowess in numbers for silly things like card tricks.