Shuffle this deck once and one thing about it stays perfectly predictable
Arrange a deck in a strict repeating four-suit pattern, then have a spectator cut it once and riffle-shuffle it together with itself, just a single time. Deal the shuffled deck off in groups of four cards. What can you guarantee will be true about every single group, no matter how the cut or shuffle fell?
Reveal the answer
Every group of four will always contain one card of each suit. Mathematician and magician Norman Gilbreath discovered this in 1958: an ordinary riffle shuffle preserves far more structure than it looks like it destroys, because it only interleaves two piles without disturbing either one's internal order. Martin Gardner popularized the trick, and the math behind it, in his Scientific American column.
— Martin Gardner, Mathematical Games: The Gilbreath Principle — Scientific American, 1960