It's a new year, but there are still a few more months in Grade -1. In this episode, I'll use clocks to explain an interesting formula that can test whether any number is prime or composite. Special thanks to my Patreon supporters at (top tier supporters are listed in this video's credits and all supporters are listed below) After you watch this video, check out my bonus video about shortcuts for testing prime numbers if you were doing it yourself with a calculator: Special thanks to: Tybie Fitzhugh, Henry Spencer, Mitch Harding, YbabFlow, Plenty W, Quinn Moyer, Julius 420, Philip Rogers, Ilmori Fajt, Brandon, August Taub, Ira Sanborn, Matthew Chudleigh, Cornelis Van Der Bent, Craig Butz, Mark S, Thorbjorn M H, Mathias Ermatinger, Edward Clarke, and Christopher Masto, Joshua S, Max, Joost Doesberg, Adam, Chris Reisenbichler, and Stan Seibert. Disclaimer: Do not copy any physical actions you see in Combo Class episodes, including any use of fire, tools, or other science experiments. COMBO CLASS LINKS: Patreon: Bonus channel: @domotro Reddit: Discord: Some topics mentioned in this video include: prime numbers, factorials, clocks, modular arithmetic congruences, divisibility/remainders, Wilson’s theorem, Wilson primes, and more. Stay tuned for next episode, which is a more casual science break where I do some bubble experiments and the rain makes my classroom flood. If you're reading this, leave a comment about clocks.