Here’s a question: how many times do you have to riffle a deck of cards before it is completely shuffled? It’s a tricky one, but math has us covered: you need seven riffles.

We can calculate the number of orderings of a deck of cards using the notion of a permutation. To find all arrangements of 52 cards in a deck, we compute 52!, which happens to be a really big number.

Riffle seven times and you’ll have a sufficiently random ordering of cards, an ordering that has likely never existed before. In other words, it’s unlikely you’ll ever shuffle two decks the same.