A card trick that works because of math, not sleight of hand
Shuffle a deck. Silently pick a number from 1 to 10, count that many cards from the top, and look at the card you land on — that's your 'key card'. Take its value (face cards count as 5) and count forward that many cards, landing on a new card, and repeat the process for as long as the deck allows. A magician, without ever seeing your choices, can usually predict exactly where you'll end up.
Reveal the answer
This is Kruskal's Count, named after mathematician Martin Kruskal and popularized by Martin Gardner. Different starting points tend to converge onto the same sequence of cards well before the deck runs out, the same way two rivers merge into one — so a performer running their own count from a fixed starting point lands on your final card the overwhelming majority of the time. It's a real demonstration of Markov chain convergence disguised as a magic trick.
— Martin Kruskal, Kruskal's count — Published by Karl Fulves and Martin Gardner, 1975