Puzzles
No matter how scrambled, any Rubik's Cube can always be solved in 20 moves or fewer
There are over 43 quintillion possible scrambles of a Rubik's Cube, and yet mathematicians proved every single one can be undone in at most 20 face turns — a number puzzlers nicknamed 'God's Number.' How could researchers possibly prove that, without checking each position one at a time?
Reveal the answer
In 2010, a team led by Tomas Rokicki used Google's spare computing power to group the 43,252,003,274,489,856,000 positions into around 2.2 billion sets of equivalent positions, then proved every set could be cleared in 20 moves or fewer — confirming 20 as the cube's 'God's Number' under the standard face-turn metric.