If the universe aligns correctly, you can decode any (dial) Master Combination lock in 8 tries or less. This video is purely practical and not explanatory. To find out why this works look at the video Samy Kamkar made: https://www.youtube.com/watch?v=qkolWO6pAL8
There are apps that will do the math for you, but it kind of takes away from the novelty of the trick when you're holding a phone! Use your head, it's fun! I also think it's important to have an idea of "why" this works, if you want reproducible effects.